diff --git a/__test__/features/Events/EventModal.test.tsx b/__test__/features/Events/EventModal.test.tsx
index 930c0a0..3f0f042 100644
--- a/__test__/features/Events/EventModal.test.tsx
+++ b/__test__/features/Events/EventModal.test.tsx
@@ -312,11 +312,15 @@ describe("EventPopover", () => {
         new Date(receivedPayload.newEvent.start)
       ).split("T")[0]
     ).toBe(formatDateToYYYYMMDDTHHMMSS(new Date(newEvent.start)).split("T")[0]);
+
+    const expectedAllDayEnd = new Date(newEvent.end);
+    expectedAllDayEnd.setUTCDate(expectedAllDayEnd.getUTCDate() + 1);
+
     expect(
       formatDateToYYYYMMDDTHHMMSS(
         new Date(receivedPayload.newEvent.end || new Date())
       ).split("T")[0]
-    ).toBe(formatDateToYYYYMMDDTHHMMSS(new Date(newEvent.end)).split("T")[0]);
+    ).toBe(formatDateToYYYYMMDDTHHMMSS(expectedAllDayEnd).split("T")[0]);
     expect(receivedPayload.newEvent.location).toBe(newEvent.location);
     expect(receivedPayload.newEvent.organizer).toEqual(newEvent.organizer);
     expect(receivedPayload.newEvent.color).toEqual(
diff --git a/src/components/Event/EventFormFields.tsx b/src/components/Event/EventFormFields.tsx
index 20f6753..415caa4 100644
--- a/src/components/Event/EventFormFields.tsx
+++ b/src/components/Event/EventFormFields.tsx
@@ -104,6 +104,7 @@ interface EventFormFieldsProps {
   // Validation
   onValidationChange?: (isValid: boolean) => void;
   showValidationErrors?: boolean;
+  onHasEndDateChangedChange?: (has: boolean) => void;
 }
 
 export default function EventFormFields({
@@ -152,6 +153,7 @@ export default function EventFormFields({
   onCalendarChange,
   onValidationChange,
   showValidationErrors = false,
+  onHasEndDateChangedChange,
 }: EventFormFieldsProps) {
   // Internal state for 4 separate fields
   const [startDate, setStartDate] = React.useState("");
@@ -159,8 +161,20 @@ export default function EventFormFields({
   const [endDate, setEndDate] = React.useState("");
   const [endTime, setEndTime] = React.useState("");
 
-  // UI state for showing/hiding end date field
-  // keep local flag if needed for future UI toggles (not used now)
+  // Track if user has manually changed end date in extended mode
+  const [hasEndDateChanged, setHasEndDateChanged] = React.useState(false);
+
+  // Reset hasEndDateChanged when modal closes
+  React.useEffect(() => {
+    if (!isOpen) {
+      setHasEndDateChanged(false);
+    }
+  }, [isOpen]);
+
+  // Notify parent about end date change visibility state
+  React.useEffect(() => {
+    onHasEndDateChangedChange?.(hasEndDateChanged);
+  }, [hasEndDateChanged, onHasEndDateChangedChange]);
 
   // Use all-day toggle hook
   const { handleAllDayToggle } = useAllDayToggle({
@@ -275,6 +289,10 @@ export default function EventFormFields({
   const handleEndDateChange = React.useCallback(
     (newDate: string) => {
       setEndDate(newDate);
+      // Track if user changed end date in extended mode
+      if (showMore) {
+        setHasEndDateChanged(true);
+      }
       const newEnd = combineDateTime(newDate, endTime);
       if (onEndChange) {
         onEndChange(newEnd);
@@ -282,7 +300,7 @@ export default function EventFormFields({
         setEnd(newEnd);
       }
     },
-    [endTime, onEndChange, setEnd]
+    [endTime, onEndChange, setEnd, showMore]
   );
 
   const handleEndTimeChange = React.useCallback(
@@ -308,6 +326,8 @@ export default function EventFormFields({
       endTime,
       allday,
       showValidationErrors,
+      hasEndDateChanged,
+      showMore,
     });
   }, [
     title,
@@ -317,6 +337,8 @@ export default function EventFormFields({
     endTime,
     allday,
     showValidationErrors,
+    hasEndDateChanged,
+    showMore,
   ]);
 
   // Notify parent about validation changes
@@ -457,6 +479,7 @@ export default function EventFormFields({
           endTime={endTime}
           allday={allday}
           showMore={showMore}
+          hasEndDateChanged={hasEndDateChanged}
           validation={validation}
           onStartDateChange={handleStartDateChange}
           onStartTimeChange={handleStartTimeChange}
@@ -606,7 +629,7 @@ export default function EventFormFields({
           )}
           <Select
             labelId="calendar-select-label"
-            value={calendarid}
+            value={calendarid ?? ""}
             label={!showMore ? "Calendar" : ""}
             displayEmpty
             onChange={(e: SelectChangeEvent) =>
diff --git a/src/components/Event/components/DateTimeFields.tsx b/src/components/Event/components/DateTimeFields.tsx
index 65465c3..ac496f0 100644
--- a/src/components/Event/components/DateTimeFields.tsx
+++ b/src/components/Event/components/DateTimeFields.tsx
@@ -3,10 +3,13 @@ import { Box, Typography } from "@mui/material";
 import { DatePicker } from "@mui/x-date-pickers/DatePicker";
 import { TimePicker } from "@mui/x-date-pickers/TimePicker";
 import { LocalizationProvider } from "@mui/x-date-pickers/LocalizationProvider";
-import { AdapterMoment } from "@mui/x-date-pickers/AdapterMoment";
-import moment, { Moment } from "moment";
+import { AdapterDayjs } from "@mui/x-date-pickers/AdapterDayjs";
+import dayjs, { Dayjs } from "dayjs";
+import customParseFormat from "dayjs/plugin/customParseFormat";
 import { LONG_DATE_FORMAT } from "../utils/dateTimeFormatters";
 
+dayjs.extend(customParseFormat);
+
 /**
  * Props for DateTimeFields component
  */
@@ -17,6 +20,7 @@ export interface DateTimeFieldsProps {
   endTime: string;
   allday: boolean;
   showMore: boolean;
+  hasEndDateChanged: boolean;
   showEndDate: boolean;
   onToggleEndDate: () => void;
   validation: {
@@ -41,6 +45,7 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
   endTime,
   allday,
   showMore,
+  hasEndDateChanged,
   showEndDate,
   onToggleEndDate,
   validation,
@@ -51,23 +56,33 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
 }) => {
   const isExpanded = showMore;
   const shouldShowEndDateNormal = allday || !!showEndDate;
+  const shouldShowFullFieldsInNormal = !allday && hasEndDateChanged;
+  const showSingleDateField =
+    !isExpanded && !shouldShowEndDateNormal && !shouldShowFullFieldsInNormal;
+  const startDateLabel = showSingleDateField ? "Date" : "Start Date";
+
   return (
-    <LocalizationProvider dateAdapter={AdapterMoment} adapterLocale="en">
+    <LocalizationProvider dateAdapter={AdapterDayjs} adapterLocale="en">
       <Box
         display="flex"
         flexDirection="column"
         sx={{ maxWidth: showMore ? "calc(100% - 145px)" : "100%" }}
       >
-        {isExpanded ? (
+        {isExpanded || shouldShowFullFieldsInNormal ? (
           <>
             <Box display="flex" gap={1} flexDirection="row" alignItems="center">
-              <Box sx={{ maxWidth: "280px", width: "45%" }}>
+              <Box sx={{ maxWidth: "300px", width: "48%" }}>
                 <DatePicker
                   label="Start Date"
                   format={LONG_DATE_FORMAT}
-                  value={startDate ? moment(startDate) : null}
-                  onChange={(newValue: Moment | null) => {
-                    onStartDateChange(newValue?.format("YYYY-MM-DD") || "");
+                  value={startDate ? dayjs(startDate) : null}
+                  onChange={(newValue) => {
+                    const value = newValue as Dayjs | null;
+                    if (!value || !value.isValid()) {
+                      return;
+                    }
+                    const formatted = value.format("YYYY-MM-DD");
+                    onStartDateChange(formatted);
                   }}
                   slotProps={{
                     textField: {
@@ -86,9 +101,14 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
                   <TimePicker
                     label="Start Time"
                     ampm={false}
-                    value={startTime ? moment(startTime, "HH:mm") : null}
-                    onChange={(newValue: Moment | null) => {
-                      onStartTimeChange(newValue?.format("HH:mm") || "");
+                    value={startTime ? dayjs(startTime, "HH:mm") : null}
+                    onChange={(newValue) => {
+                      const value = newValue as Dayjs | null;
+                      if (!value || !value.isValid()) {
+                        return;
+                      }
+                      const formatted = value?.format("HH:mm") || "";
+                      onStartTimeChange(formatted);
                     }}
                     slotProps={{
                       textField: {
@@ -105,13 +125,18 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
               )}
             </Box>
             <Box display="flex" gap={1} flexDirection="row" alignItems="center">
-              <Box sx={{ maxWidth: "280px", width: "45%" }}>
+              <Box sx={{ maxWidth: "300px", width: "48%" }}>
                 <DatePicker
                   label="End Date"
                   format={LONG_DATE_FORMAT}
-                  value={endDate ? moment(endDate) : null}
-                  onChange={(newValue: Moment | null) => {
-                    onEndDateChange(newValue?.format("YYYY-MM-DD") || "");
+                  value={endDate ? dayjs(endDate) : null}
+                  onChange={(newValue) => {
+                    const value = newValue as Dayjs | null;
+                    if (!value || !value.isValid()) {
+                      return;
+                    }
+                    const formatted = value.format("YYYY-MM-DD");
+                    onEndDateChange(formatted);
                   }}
                   slotProps={{
                     textField: {
@@ -131,9 +156,14 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
                   <TimePicker
                     label="End Time"
                     ampm={false}
-                    value={endTime ? moment(endTime, "HH:mm") : null}
-                    onChange={(newValue: Moment | null) => {
-                      onEndTimeChange(newValue?.format("HH:mm") || "");
+                    value={endTime ? dayjs(endTime, "HH:mm") : null}
+                    onChange={(newValue) => {
+                      const value = newValue as Dayjs | null;
+                      if (!value || !value.isValid()) {
+                        return;
+                      }
+                      const formatted = value?.format("HH:mm") || "";
+                      onEndTimeChange(formatted);
                     }}
                     slotProps={{
                       textField: {
@@ -153,13 +183,18 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
           </>
         ) : shouldShowEndDateNormal ? (
           <Box display="flex" gap={1} flexDirection="row" alignItems="center">
-            <Box sx={{ maxWidth: "280px", width: "45%" }}>
+            <Box sx={{ maxWidth: "300px", width: "48%" }}>
               <DatePicker
                 label="Start Date"
                 format={LONG_DATE_FORMAT}
-                value={startDate ? moment(startDate) : null}
-                onChange={(newValue: Moment | null) => {
-                  onStartDateChange(newValue?.format("YYYY-MM-DD") || "");
+                value={startDate ? dayjs(startDate) : null}
+                onChange={(newValue) => {
+                  const value = newValue as Dayjs | null;
+                  if (!value || !value.isValid()) {
+                    return;
+                  }
+                  const formatted = value.format("YYYY-MM-DD");
+                  onStartDateChange(formatted);
                 }}
                 slotProps={{
                   textField: {
@@ -173,13 +208,18 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
                 }}
               />
             </Box>
-            <Box sx={{ maxWidth: "280px", width: "45%" }}>
+            <Box sx={{ maxWidth: "300px", width: "48%" }}>
               <DatePicker
                 label="End Date"
                 format={LONG_DATE_FORMAT}
-                value={endDate ? moment(endDate) : null}
-                onChange={(newValue: Moment | null) => {
-                  onEndDateChange(newValue?.format("YYYY-MM-DD") || "");
+                value={endDate ? dayjs(endDate) : null}
+                onChange={(newValue) => {
+                  const value = newValue as Dayjs | null;
+                  if (!value || !value.isValid()) {
+                    return;
+                  }
+                  const formatted = value.format("YYYY-MM-DD");
+                  onEndDateChange(formatted);
                 }}
                 slotProps={{
                   textField: {
@@ -197,13 +237,18 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
           </Box>
         ) : (
           <Box display="flex" gap={1} flexDirection="row" alignItems="center">
-            <Box sx={{ maxWidth: "280px", width: "45%" }}>
+            <Box sx={{ maxWidth: "300px", width: "48%" }}>
               <DatePicker
-                label="Start Date"
+                label={startDateLabel}
                 format={LONG_DATE_FORMAT}
-                value={startDate ? moment(startDate) : null}
-                onChange={(newValue: Moment | null) => {
-                  onStartDateChange(newValue?.format("YYYY-MM-DD") || "");
+                value={startDate ? dayjs(startDate) : null}
+                onChange={(newValue) => {
+                  const value = newValue as Dayjs | null;
+                  if (!value || !value.isValid()) {
+                    return;
+                  }
+                  const formatted = value.format("YYYY-MM-DD");
+                  onStartDateChange(formatted);
                 }}
                 slotProps={{
                   textField: {
@@ -221,9 +266,14 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
               <TimePicker
                 label="Start Time"
                 ampm={false}
-                value={startTime ? moment(startTime, "HH:mm") : null}
-                onChange={(newValue: Moment | null) => {
-                  onStartTimeChange(newValue?.format("HH:mm") || "");
+                value={startTime ? dayjs(startTime, "HH:mm") : null}
+                onChange={(newValue) => {
+                  const value = newValue as Dayjs | null;
+                  if (!value || !value.isValid()) {
+                    return;
+                  }
+                  const formatted = value?.format("HH:mm") || "";
+                  onStartTimeChange(formatted);
                 }}
                 disabled={allday}
                 slotProps={{
@@ -242,9 +292,14 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
               <TimePicker
                 label="End Time"
                 ampm={false}
-                value={endTime ? moment(endTime, "HH:mm") : null}
-                onChange={(newValue: Moment | null) => {
-                  onEndTimeChange(newValue?.format("HH:mm") || "");
+                value={endTime ? dayjs(endTime, "HH:mm") : null}
+                onChange={(newValue) => {
+                  const value = newValue as Dayjs | null;
+                  if (!value || !value.isValid()) {
+                    return;
+                  }
+                  const formatted = value?.format("HH:mm") || "";
+                  onEndTimeChange(formatted);
                 }}
                 disabled={allday}
                 slotProps={{
@@ -266,7 +321,11 @@ export const DateTimeFields: React.FC<DateTimeFieldsProps> = ({
         {/* Second row: Error message */}
         {validation.errors.dateTime && (
           <Box display="flex" gap={1} flexDirection="row">
-            <Box sx={{ width: isExpanded ? "0" : allday ? "45%" : "64%" }} />
+            <Box
+              sx={{
+                width: "1%",
+              }}
+            />
             <Box>
               <Typography variant="caption" sx={{ color: "error.main" }}>
                 {validation.errors.dateTime}
diff --git a/src/components/Event/utils/formValidation.ts b/src/components/Event/utils/formValidation.ts
index 1569406..45db7b6 100644
--- a/src/components/Event/utils/formValidation.ts
+++ b/src/components/Event/utils/formValidation.ts
@@ -11,6 +11,8 @@ export interface ValidationParams {
   endTime: string;
   allday: boolean;
   showValidationErrors: boolean;
+  hasEndDateChanged?: boolean;
+  showMore?: boolean;
 }
 
 /**
@@ -38,6 +40,8 @@ export function validateEventForm(params: ValidationParams): ValidationResult {
     endTime,
     allday,
     showValidationErrors,
+    hasEndDateChanged = false,
+    showMore = false,
   } = params;
 
   const isTitleValid = title.trim().length > 0;
@@ -46,6 +50,11 @@ export function validateEventForm(params: ValidationParams): ValidationResult {
   let isDateTimeValid = true;
   let dateTimeError = "";
 
+  // Determine which fields are visible based on UI mode
+  const showFullFields = showMore || hasEndDateChanged;
+  const showTimeOnly = !allday && !showFullFields;
+  const showDateOnly = allday;
+
   // Validate start date
   if (!startDate || startDate.trim() === "") {
     isDateTimeValid = false;
@@ -54,21 +63,78 @@ export function validateEventForm(params: ValidationParams): ValidationResult {
   // Validate start time (if not all-day)
   else if (!allday && (!startTime || startTime.trim() === "")) {
     isDateTimeValid = false;
-    dateTimeError = "Start time and End time is required";
+    dateTimeError = "Start time is required";
   }
-  // Validate end date
-  else if (!endDate || endDate.trim() === "") {
-    isDateTimeValid = false;
-    dateTimeError = "End date is required";
-  }
-  // Validate end time (if not all-day)
-  else if (!allday && (!endTime || endTime.trim() === "")) {
-    isDateTimeValid = false;
-    dateTimeError = "End time is required";
-  }
-  // Validate end vs start
-  else {
-    if (allday) {
+  // Validate end fields based on UI mode
+  else if (showFullFields) {
+    // 4 fields mode: validate both end date and end time
+    if (!endDate || endDate.trim() === "") {
+      isDateTimeValid = false;
+      dateTimeError = "End date is required";
+    } else if (!allday && (!endTime || endTime.trim() === "")) {
+      isDateTimeValid = false;
+      dateTimeError = "End time is required";
+    } else {
+      // Validate total datetime
+      if (allday) {
+        const toLocalDate = (ymd: string) => {
+          const [y, m, d] = ymd.split("-").map((v) => parseInt(v, 10));
+          if (!y || !m || !d) return new Date(NaN);
+          return new Date(y, m - 1, d);
+        };
+
+        const startOnly = toLocalDate(startDate);
+        const endOnly = toLocalDate(endDate);
+
+        if (isNaN(startOnly.getTime()) || isNaN(endOnly.getTime())) {
+          isDateTimeValid = false;
+          dateTimeError = "Invalid date";
+        } else if (endOnly < startOnly) {
+          isDateTimeValid = false;
+          dateTimeError = "End date must be on or after start date";
+        }
+      } else {
+        const startDateTime = new Date(combineDateTime(startDate, startTime));
+        const endDateTime = new Date(combineDateTime(endDate, endTime));
+
+        if (isNaN(startDateTime.getTime()) || isNaN(endDateTime.getTime())) {
+          isDateTimeValid = false;
+          dateTimeError = "Invalid date/time";
+        } else if (endDateTime <= startDateTime) {
+          isDateTimeValid = false;
+          dateTimeError = "End date/time must be after start date/time";
+        }
+      }
+    }
+  } else if (showTimeOnly) {
+    // 3 fields mode: validate time only (end time > start time, same day)
+    if (!endTime || endTime.trim() === "") {
+      isDateTimeValid = false;
+      dateTimeError = "End time is required";
+    } else {
+      // Compare times only (same day is assumed)
+      const startTimeParts = startTime.split(":");
+      const endTimeParts = endTime.split(":");
+      if (startTimeParts.length === 2 && endTimeParts.length === 2) {
+        const startMinutes =
+          parseInt(startTimeParts[0]) * 60 + parseInt(startTimeParts[1]);
+        const endMinutes =
+          parseInt(endTimeParts[0]) * 60 + parseInt(endTimeParts[1]);
+        if (endMinutes <= startMinutes) {
+          isDateTimeValid = false;
+          dateTimeError = "End time must be after start time";
+        }
+      } else {
+        isDateTimeValid = false;
+        dateTimeError = "Invalid time format";
+      }
+    }
+  } else if (showDateOnly) {
+    // 2 fields mode: validate date only
+    if (!endDate || endDate.trim() === "") {
+      isDateTimeValid = false;
+      dateTimeError = "End date is required";
+    } else {
       const toLocalDate = (ymd: string) => {
         const [y, m, d] = ymd.split("-").map((v) => parseInt(v, 10));
         if (!y || !m || !d) return new Date(NaN);
@@ -85,17 +151,6 @@ export function validateEventForm(params: ValidationParams): ValidationResult {
         isDateTimeValid = false;
         dateTimeError = "End date must be on or after start date";
       }
-    } else {
-      const startDateTime = new Date(combineDateTime(startDate, startTime));
-      const endDateTime = new Date(combineDateTime(endDate, endTime));
-
-      if (isNaN(startDateTime.getTime()) || isNaN(endDateTime.getTime())) {
-        isDateTimeValid = false;
-        dateTimeError = "Invalid date/time";
-      } else if (endDateTime <= startDateTime) {
-        isDateTimeValid = false;
-        dateTimeError = "End time must be after start time";
-      }
     }
   }
 
diff --git a/src/features/Events/EventModal.tsx b/src/features/Events/EventModal.tsx
index 33cb0f4..f34a67d 100644
--- a/src/features/Events/EventModal.tsx
+++ b/src/features/Events/EventModal.tsx
@@ -29,6 +29,7 @@ import {
   formatLocalDateTime,
   formatDateTimeInTimezone,
 } from "../../components/Event/utils/dateTimeFormatters";
+import { addDays } from "../../components/Event/utils/dateRules";
 
 function EventPopover({
   anchorEl,
@@ -96,7 +97,7 @@ function EventPopover({
   const [start, setStart] = useState(event?.start ? event.start : "");
   const [end, setEnd] = useState(event?.end ? event.end : "");
   const [calendarid, setCalendarid] = useState(
-    event?.calId ?? userPersonnalCalendars[0]?.id
+    event?.calId ?? userPersonnalCalendars[0]?.id ?? ""
   );
   const [allday, setAllDay] = useState(event?.allday ?? false);
   const [repetition, setRepetition] = useState<RepetitionObject>(
@@ -121,6 +122,7 @@ function EventPopover({
   );
   const [isFormValid, setIsFormValid] = useState(false);
   const [showValidationErrors, setShowValidationErrors] = useState(false);
+  const [hasEndDateChanged, setHasEndDateChanged] = useState(false);
 
   // Use ref to track if we've already initialized to avoid infinite loop
   const isInitializedRef = useRef(false);
@@ -147,7 +149,13 @@ function EventPopover({
     setLocation("");
     setStart("");
     setEnd("");
-    setCalendarid(userPersonnalCalendars[0]?.id);
+    if (
+      userPersonnalCalendars &&
+      userPersonnalCalendars.length > 0 &&
+      userPersonnalCalendars[0]?.id
+    ) {
+      setCalendarid(userPersonnalCalendars[0].id);
+    }
     setAllDay(false);
     setRepetition({} as RepetitionObject);
     setAlarm("");
@@ -156,7 +164,7 @@ function EventPopover({
     setTimezone(calendarTimezone);
     setHasVideoConference(false);
     setMeetingLink(null);
-  }, [calendarTimezone]);
+  }, [calendarTimezone, userPersonnalCalendars]);
 
   // Track if we should sync from selectedRange (only on initial selection, not on toggle)
   const shouldSyncFromRangeRef = useRef(true);
@@ -224,9 +232,38 @@ function EventPopover({
         const startValue = selectedRange.allDay
           ? startStr.split("T")[0]
           : startStr.slice(0, 16); // YYYY-MM-DDTHH:mm
-        const endValue = selectedRange.allDay
+        let endValue = selectedRange.allDay
           ? endStr.split("T")[0]
           : endStr.slice(0, 16);
+
+        // For all-day slots: detect single click vs drag multiple days
+        // FullCalendar uses exclusive end, so end date = actual end date + 1
+        // For UI display, we need to adjust:
+        // - Single click (end = start + 1): set end = start for UI
+        // - Drag multiple days (end = actual end + 1): set end = end - 1 for UI
+        if (selectedRange.allDay) {
+          const startDateOnly = startValue.slice(0, 10);
+          const endDateOnlyFromRange = endValue.slice(0, 10);
+
+          // Calculate days difference
+          const startDateObj = new Date(startDateOnly);
+          const endDateObj = new Date(endDateOnlyFromRange);
+          const daysDiff = Math.floor(
+            (endDateObj.getTime() - startDateObj.getTime()) /
+              (1000 * 60 * 60 * 24)
+          );
+
+          if (daysDiff <= 1) {
+            // Single click: FullCalendar gives end = start + 1, set end = start for UI
+            endValue = startValue;
+          } else {
+            // Drag multiple days: FullCalendar gives end = actual end + 1, subtract 1 for UI
+            const adjustedEndDate = new Date(endDateObj);
+            adjustedEndDate.setDate(adjustedEndDate.getDate() - 1);
+            endValue = adjustedEndDate.toISOString().split("T")[0];
+          }
+        }
+
         setStart(startValue);
         setEnd(endValue);
 
@@ -240,7 +277,42 @@ function EventPopover({
         // Fallback: format Date objects using local time components
         // Only set if both start and end are valid
         const formattedStart = formatLocalDateTime(selectedRange.start);
-        const formattedEnd = formatLocalDateTime(selectedRange.end);
+        let formattedEnd = formatLocalDateTime(selectedRange.end);
+
+        // For all-day slots: detect single click vs drag multiple days
+        // FullCalendar uses exclusive end, so end date = actual end date + 1
+        // For UI display, we need to adjust:
+        // - Single click (end = start + 1): set end = start for UI
+        // - Drag multiple days (end = actual end + 1): set end = end - 1 for UI
+        if (selectedRange.allDay && formattedStart && formattedEnd) {
+          const startDateOnly = formattedStart.slice(0, 10);
+          const endDateOnly = formattedEnd.slice(0, 10);
+
+          // Calculate days difference
+          const startDateObj = new Date(startDateOnly);
+          const endDateObj = new Date(endDateOnly);
+          const daysDiff = Math.floor(
+            (endDateObj.getTime() - startDateObj.getTime()) /
+              (1000 * 60 * 60 * 24)
+          );
+
+          if (daysDiff <= 1) {
+            // Single click: FullCalendar gives end = start + 1, set end = start for UI
+            formattedEnd = formattedStart;
+          } else {
+            // Drag multiple days: FullCalendar gives end = actual end + 1, subtract 1 for UI
+            const adjustedEndDate = new Date(endDateObj);
+            adjustedEndDate.setDate(adjustedEndDate.getDate() - 1);
+            const adjustedEndDateStr = adjustedEndDate
+              .toISOString()
+              .split("T")[0];
+            // Preserve time part if exists, otherwise use date only
+            formattedEnd = formattedEnd.includes("T")
+              ? `${adjustedEndDateStr}T${formattedEnd.split("T")[1]}`
+              : adjustedEndDateStr;
+          }
+        }
+
         if (formattedStart) setStart(formattedStart);
         if (formattedEnd) setEnd(formattedEnd);
         if (formattedStart && formattedEnd) {
@@ -323,7 +395,13 @@ function EventPopover({
         setEnd("");
       }
 
-      setCalendarid(userPersonnalCalendars[0].id);
+      if (
+        userPersonnalCalendars &&
+        userPersonnalCalendars.length > 0 &&
+        userPersonnalCalendars[0]?.id
+      ) {
+        setCalendarid(userPersonnalCalendars[0].id);
+      }
       setRepetition(event.repetition ?? ({} as RepetitionObject));
       setShowRepeat(event.repetition?.freq ? true : false);
       setAttendees(
@@ -378,7 +456,13 @@ function EventPopover({
       setDescription("");
       setAttendees([]);
       setLocation("");
-      setCalendarid(userPersonnalCalendars[0].id);
+      if (
+        userPersonnalCalendars &&
+        userPersonnalCalendars.length > 0 &&
+        userPersonnalCalendars[0]?.id
+      ) {
+        setCalendarid(userPersonnalCalendars[0].id);
+      }
       setAllDay(false);
       setRepetition({} as RepetitionObject);
       setAlarm("");
@@ -407,7 +491,9 @@ function EventPopover({
             startStr: newStart,
             allDay: allday,
           } as DateSelectArg;
-          calendarRef.current?.select(newRange);
+          setTimeout(() => {
+            calendarRef.current?.select(newRange);
+          }, 0);
           return newRange;
         });
       });
@@ -430,7 +516,9 @@ function EventPopover({
             endStr: newEnd,
             allDay: allday,
           } as DateSelectArg;
-          calendarRef.current?.select(newRange);
+          setTimeout(() => {
+            calendarRef.current?.select(newRange);
+          }, 0);
           return newRange;
         });
       });
@@ -462,7 +550,9 @@ function EventPopover({
             ),
             allDay: newAllDay,
           } as DateSelectArg;
-          calendarRef.current?.select(newRange);
+          setTimeout(() => {
+            calendarRef.current?.select(newRange);
+          }, 0);
           return newRange;
         });
       });
@@ -491,10 +581,20 @@ function EventPopover({
     }
     const newEventUID = crypto.randomUUID();
 
+    // Resolve target calendar safely
+    const targetCalendar: Calendars | undefined =
+      calList[calendarid] ||
+      userPersonnalCalendars[0] ||
+      (Object.values(calList)[0] as Calendars | undefined);
+    if (!targetCalendar || !targetCalendar.id) {
+      console.error("No target calendar available to save event");
+      return;
+    }
+
     const newEvent: CalendarEvent = {
-      calId: calList[calendarid].id,
+      calId: targetCalendar.id,
       title,
-      URL: `/calendars/${calList[calendarid].id}/${newEventUID}.ics`,
+      URL: `/calendars/${targetCalendar.id}/${newEventUID}.ics`,
       start: "",
       allday,
       uid: newEventUID,
@@ -515,22 +615,35 @@ function EventPopover({
         },
       ],
       transp: busy,
-      color: calList[calendarid]?.color,
+      color: targetCalendar?.color,
       alarm: { trigger: alarm, action: "EMAIL" },
       x_openpass_videoconference: meetingLink || undefined,
     };
 
     if (allday) {
       const startDateOnly = (start || "").split("T")[0];
-      const endDateOnlyBase = (end || start || "").split("T")[0];
+      const endDateOnlyUI = (end || start || "").split("T")[0];
+      // For all-day events, API needs end date = UI end date + 1 day
+      const endDateOnlyAPI = addDays(endDateOnlyUI, 1);
       const startDateObj = new Date(`${startDateOnly}T00:00:00`);
-      const endDateObj = new Date(`${endDateOnlyBase}T00:00:00`);
+      const endDateObj = new Date(`${endDateOnlyAPI}T00:00:00`);
       newEvent.start = startDateObj.toISOString();
       newEvent.end = endDateObj.toISOString();
     } else {
       newEvent.start = new Date(start).toISOString();
       if (end) {
-        newEvent.end = new Date(end).toISOString();
+        // In normal mode, only override end date when the end date field is not shown
+        if (!showMore && !hasEndDateChanged) {
+          const startDateOnly = (start || "").split("T")[0];
+          const endTimeOnly = end.includes("T")
+            ? end.split("T")[1]?.slice(0, 5) || "00:00"
+            : "00:00";
+          const endDateTime = `${startDateOnly}T${endTimeOnly}`;
+          newEvent.end = new Date(endDateTime).toISOString();
+        } else {
+          // Extended mode or end date explicitly shown in normal mode: use actual end datetime
+          newEvent.end = new Date(end).toISOString();
+        }
       }
     }
 
@@ -550,7 +663,7 @@ function EventPopover({
     // Save to API in background
     await dispatch(
       putEventAsync({
-        cal: calList[calendarid],
+        cal: targetCalendar,
         newEvent,
       })
     );
@@ -633,6 +746,7 @@ function EventPopover({
         onAllDayChange={handleAllDayChange}
         onValidationChange={setIsFormValid}
         showValidationErrors={showValidationErrors}
+        onHasEndDateChangedChange={setHasEndDateChanged}
       />
     </ResponsiveDialog>
   );
diff --git a/src/features/Events/EventUpdateModal.tsx b/src/features/Events/EventUpdateModal.tsx
index 52cc63a..dd655e7 100644
--- a/src/features/Events/EventUpdateModal.tsx
+++ b/src/features/Events/EventUpdateModal.tsx
@@ -19,6 +19,7 @@ import { TIMEZONES } from "../../utils/timezone-data";
 import { addVideoConferenceToDescription } from "../../utils/videoConferenceUtils";
 import EventFormFields from "../../components/Event/EventFormFields";
 import { formatDateTimeInTimezone } from "../../components/Event/utils/dateTimeFormatters";
+import { addDays } from "../../components/Event/utils/dateRules";
 import { getEvent, deleteEvent, putEvent } from "./EventApi";
 import { refreshCalendars } from "../../components/Event/utils/eventUtils";
 import { getCalendarRange } from "../../utils/dateUtils";
@@ -154,7 +155,7 @@ function EventUpdateModal({
   );
   const [newCalId, setNewCalId] = useState(calId);
   const [calendarid, setCalendarid] = useState(
-    calId ?? userPersonnalCalendars[0]?.id
+    calId ?? userPersonnalCalendars[0]?.id ?? ""
   );
 
   const [attendees, setAttendees] = useState<userAttendee[]>([]);
@@ -162,6 +163,7 @@ function EventUpdateModal({
   const [meetingLink, setMeetingLink] = useState<string | null>(null);
   const [isFormValid, setIsFormValid] = useState(false);
   const [showValidationErrors, setShowValidationErrors] = useState(false);
+  const [hasEndDateChanged, setHasEndDateChanged] = useState(false);
 
   const resetAllStateToDefault = useCallback(() => {
     setShowMore(false);
@@ -379,12 +381,27 @@ function EventUpdateModal({
       // For single events or "solo" edits, use the edited dates from form
       if (allday) {
         // For all-day events, use date format (YYYY-MM-DD)
-        startDate = new Date(start).toISOString().split("T")[0];
-        endDate = new Date(end).toISOString().split("T")[0];
+        // API needs end date = UI end date + 1 day
+        const startDateOnly = new Date(start).toISOString().split("T")[0];
+        const endDateOnlyUI = new Date(end).toISOString().split("T")[0];
+        const endDateOnlyAPI = addDays(endDateOnlyUI, 1);
+        startDate = startDateOnly;
+        endDate = endDateOnlyAPI;
       } else {
-        // For timed events, use full datetime
+        // For timed events
         startDate = new Date(start).toISOString();
-        endDate = new Date(end).toISOString();
+        // In normal mode, only override end date when the end date field is not shown
+        if (!showMore && !hasEndDateChanged) {
+          const startDateOnly = (start || "").split("T")[0];
+          const endTimeOnly = end.includes("T")
+            ? end.split("T")[1]?.slice(0, 5) || "00:00"
+            : "00:00";
+          const endDateTime = `${startDateOnly}T${endTimeOnly}`;
+          endDate = new Date(endDateTime).toISOString();
+        } else {
+          // Extended mode: use actual end datetime
+          endDate = new Date(end).toISOString();
+        }
       }
     }
 
@@ -753,6 +770,7 @@ function EventUpdateModal({
         }}
         onValidationChange={setIsFormValid}
         showValidationErrors={showValidationErrors}
+        onHasEndDateChangedChange={setHasEndDateChanged}
       />
     </ResponsiveDialog>
   );