fix: wrong day of week (#2468)

frontend/appflowy_flutter/lib/plugins/database_view/calendar/presentation/calendar_page.dart

@@ -171,8 +171,9 @@ class _CalendarPageState extends State<CalendarPage> {
   }
 
   Widget _headerWeekDayBuilder(day) {
+    // incoming day starts from Monday, the symbols start from Sunday
     final symbols = DateFormat.EEEE(context.locale.toLanguageTag()).dateSymbols;
-    final weekDayString = symbols.WEEKDAYS[day];
+    final weekDayString = symbols.WEEKDAYS[(day + 1) % 7];
     return Center(
       child: Padding(
         padding: CalendarSize.daysOfWeekInsets,
@@ -216,8 +217,8 @@ class _CalendarPageState extends State<CalendarPage> {
   }
 
   WeekDays _weekdayFromInt(int dayOfWeek) {
-    // MonthView places the first day of week on the second column for some reason.
-    return WeekDays.values[(dayOfWeek + 1) % 7];
+    // dayOfWeek starts from Sunday, WeekDays starts from Monday
+    return WeekDays.values[(dayOfWeek - 1) % 7];
   }
 }
 

frontend/appflowy_flutter/lib/plugins/database_view/calendar/presentation/toolbar/calendar_layout_setting.dart

@@ -340,7 +340,7 @@ class FirstDayOfWeek extends StatelessWidget {
             DateFormat.EEEE(context.locale.toLanguageTag()).dateSymbols;
         // starts from sunday
         final items = symbols.WEEKDAYS.asMap().entries.map((entry) {
-          final index = (entry.key - 1) % 7;
+          final index = entry.key;
           final string = entry.value;
           return SizedBox(
             height: GridSize.popoverItemHeight,