feat(Calendar): Add firstDayOfWeek prop

packages/@internationalized/date/docs/CalendarDate.mdx

@@ -315,6 +315,12 @@ startOfWeek(date, 'en-US'); // 2022-01-30
 startOfWeek(date, 'fr-FR'); // 2022-01-31
 ```
 
+You can also provide an optional `firstDayOfWeek` argument to override the default first day of the week determined by the locale. It accepts a week day abbreviation, e.g. `sun`, `mon`, `tue`, etc.
+
+```tsx
+startOfWeek(date, 'en-US', 'mon'); // 2022-01-31
+```
+
 ### Day of week
 
 The <TypeLink links={docs.links} type={docs.exports.getDayOfWeek} /> function returns the day of the week for the given date and locale. Days are numbered from zero to six, where zero is the first day of the week in the given locale. For example, in the United States, the first day of the week is Sunday, but in France it is Monday.
@@ -328,6 +334,12 @@ getDayOfWeek(date, 'en-US'); // 0
 getDayOfWeek(date, 'fr-FR'); // 6
 ```
 
+You can also provide an optional `firstDayOfWeek` argument to override the default first day of the week determined by the locale. It accepts a week day abbreviation, e.g. `sun`, `mon`, `tue`, etc.
+
+```tsx
+getDayOfWeek(date, 'en-US', 'mon'); // 6
+```
+
 ### Weekdays and weekends
 
 The <TypeLink links={docs.links} type={docs.exports.isWeekday} /> and <TypeLink links={docs.links} type={docs.exports.isWeekend} /> functions can be used to determine if a date is weekday or weekend respectively. This depends on the locale. For example, in the United States, weekends are Saturday and Sunday, but in Israel they are Friday and Saturday.
@@ -356,3 +368,9 @@ let date = new CalendarDate(2021, 1, 1);
 getWeeksInMonth(date, 'en-US'); // 6
 getWeeksInMonth(date, 'fr-FR'); // 5
 ```
+
+You can also provide an optional `firstDayOfWeek` argument to override the default first day of the week determined by the locale. It accepts a week day abbreviation, e.g. `sun`, `mon`, `tue`, etc.
+
+```tsx
+getWeeksInMonth(date, 'en-US', 'mon'); // 5
+```

packages/@internationalized/date/docs/CalendarDateTime.mdx

@@ -367,6 +367,12 @@ startOfWeek(date, 'en-US'); // 2022-01-30T09:45
 startOfWeek(date, 'fr-FR'); // 2022-01-31T09:45
 ```
 
+You can also provide an optional `firstDayOfWeek` argument to override the default first day of the week determined by the locale. It accepts a week day abbreviation, e.g. `sun`, `mon`, `tue`, etc.
+
+```tsx
+startOfWeek(date, 'en-US', 'mon'); // 2022-01-31T09:45
+```
+
 ### Day of week
 
 The <TypeLink links={docs.links} type={docs.exports.getDayOfWeek} /> function returns the day of the week for the given date and locale. Days are numbered from zero to six, where zero is the first day of the week in the given locale. For example, in the United States, the first day of the week is Sunday, but in France it is Monday.
@@ -380,6 +386,12 @@ getDayOfWeek(date, 'en-US'); // 0
 getDayOfWeek(date, 'fr-FR'); // 6
 ```
 
+You can also provide an optional `firstDayOfWeek` argument to override the default first day of the week determined by the locale. It accepts a week day abbreviation, e.g. `sun`, `mon`, `tue`, etc.
+
+```tsx
+getDayOfWeek(date, 'en-US', 'mon'); // 6
+```
+
 ### Weekdays and weekends
 
 The <TypeLink links={docs.links} type={docs.exports.isWeekday} /> and <TypeLink links={docs.links} type={docs.exports.isWeekend} /> functions can be used to determine if a date is weekday or weekend respectively. This depends on the locale. For example, in the United States, weekends are Saturday and Sunday, but in Israel they are Friday and Saturday.
@@ -408,3 +420,9 @@ let date = new CalendarDateTime(2021, 1, 1, 8, 30);
 getWeeksInMonth(date, 'en-US'); // 6
 getWeeksInMonth(date, 'fr-FR'); // 5
 ```
+
+You can also provide an optional `firstDayOfWeek` argument to override the default first day of the week determined by the locale. It accepts a week day abbreviation, e.g. `sun`, `mon`, `tue`, etc.
+
+```tsx
+getWeeksInMonth(date, 'en-US', 'mon'); // 5
+```

packages/@internationalized/date/docs/ZonedDateTime.mdx

@@ -475,6 +475,12 @@ startOfWeek(date, 'en-US'); // 2022-01-30T09:45[America/Los_Angeles]
 startOfWeek(date, 'fr-FR'); // 2022-01-31T09:45[America/Los_Angeles]
 ```
 
+You can also provide an optional `firstDayOfWeek` argument to override the default first day of the week determined by the locale. It accepts a week day abbreviation, e.g. `sun`, `mon`, `tue`, etc.
+
+```tsx
+startOfWeek(date, 'en-US', 'mon'); // 2022-01-31T09:45[America/Los_Angeles]
+```
+
 ### Day of week
 
 The <TypeLink links={docs.links} type={docs.exports.getDayOfWeek} /> function returns the day of the week for the given date and locale. Days are numbered from zero to six, where zero is the first day of the week in the given locale. For example, in the United States, the first day of the week is Sunday, but in France it is Monday.
@@ -488,6 +494,12 @@ getDayOfWeek(date, 'en-US'); // 0
 getDayOfWeek(locale, 'fr-FR'); // 6
 ```
 
+You can also provide an optional `firstDayOfWeek` argument to override the default first day of the week determined by the locale. It accepts a week day abbreviation, e.g. `sun`, `mon`, `tue`, etc.
+
+```tsx
+getDayOfWeek(date, 'en-US', 'mon'); // 6
+```
+
 ### Weekdays and weekends
 
 The <TypeLink links={docs.links} type={docs.exports.isWeekday} /> and <TypeLink links={docs.links} type={docs.exports.isWeekend} /> functions can be used to determine if a date is weekday or weekend respectively. This depends on the locale. For example, in the United States, weekends are Saturday and Sunday, but in Israel they are Friday and Saturday.
@@ -516,3 +528,9 @@ let date = parseZonedDateTime('2023-01-01T08:30[America/Los_Angeles]');
 getWeeksInMonth(date, 'en-US'); // 5
 getWeeksInMonth(date, 'fr-FR'); // 6
 ```
+
+You can also provide an optional `firstDayOfWeek` argument to override the default first day of the week determined by the locale. It accepts a week day abbreviation, e.g. `sun`, `mon`, `tue`, etc.
+
+```tsx
+getWeeksInMonth(date, 'en-US', 'mon'); // 6
+```

packages/@internationalized/date/src/queries.ts

@@ -64,17 +64,30 @@ export function isToday(date: DateValue, timeZone: string): boolean {
   return isSameDay(date, today(timeZone));
 }
 
+const DAY_MAP = {
+  sun: 0,
+  mon: 1,
+  tue: 2,
+  wed: 3,
+  thu: 4,
+  fri: 5,
+  sat: 6
+};
+
+type DayOfWeek = 'sun' | 'mon' | 'tue' | 'wed' | 'thu' | 'fri' | 'sat';
+
 /**
  * Returns the day of week for the given date and locale. Days are numbered from zero to six,
  * where zero is the first day of the week in the given locale. For example, in the United States,
  * the first day of the week is Sunday, but in France it is Monday.
  */
-export function getDayOfWeek(date: DateValue, locale: string): number {
+export function getDayOfWeek(date: DateValue, locale: string, firstDayOfWeek?: DayOfWeek): number {
   let julian = date.calendar.toJulianDay(date);
 
   // If julian is negative, then julian % 7 will be negative, so we adjust
   // accordingly.  Julian day 0 is Monday.
-  let dayOfWeek = Math.ceil(julian + 1 - getWeekStart(locale)) % 7;
+  let weekStart = firstDayOfWeek ? DAY_MAP[firstDayOfWeek] : getWeekStart(locale);
+  let dayOfWeek = Math.ceil(julian + 1 - weekStart) % 7;
   if (dayOfWeek < 0) {
     dayOfWeek += 7;
   }
@@ -181,22 +194,22 @@ export function getMinimumDayInMonth(date: AnyCalendarDate) {
 }
 
 /** Returns the first date of the week for the given date and locale. */
-export function startOfWeek(date: ZonedDateTime, locale: string): ZonedDateTime;
-export function startOfWeek(date: CalendarDateTime, locale: string): CalendarDateTime;
-export function startOfWeek(date: CalendarDate, locale: string): CalendarDate;
-export function startOfWeek(date: DateValue, locale: string): DateValue;
-export function startOfWeek(date: DateValue, locale: string): DateValue {
-  let dayOfWeek = getDayOfWeek(date, locale);
+export function startOfWeek(date: ZonedDateTime, locale: string, firstDayOfWeek?: DayOfWeek): ZonedDateTime;
+export function startOfWeek(date: CalendarDateTime, locale: string, firstDayOfWeek?: DayOfWeek): CalendarDateTime;
+export function startOfWeek(date: CalendarDate, locale: string, firstDayOfWeek?: DayOfWeek): CalendarDate;
+export function startOfWeek(date: DateValue, locale: string, firstDayOfWeek?: DayOfWeek): DateValue;
+export function startOfWeek(date: DateValue, locale: string, firstDayOfWeek?: DayOfWeek): DateValue {
+  let dayOfWeek = getDayOfWeek(date, locale, firstDayOfWeek);
   return date.subtract({days: dayOfWeek});
 }
 
 /** Returns the last date of the week for the given date and locale. */
-export function endOfWeek(date: ZonedDateTime, locale: string): ZonedDateTime;
-export function endOfWeek(date: CalendarDateTime, locale: string): CalendarDateTime;
-export function endOfWeek(date: CalendarDate, locale: string): CalendarDate;
-export function endOfWeek(date: DateValue, locale: string): DateValue;
-export function endOfWeek(date: DateValue, locale: string): DateValue {
-  return startOfWeek(date, locale).add({days: 6});
+export function endOfWeek(date: ZonedDateTime, locale: string, firstDayOfWeek?: DayOfWeek): ZonedDateTime;
+export function endOfWeek(date: CalendarDateTime, locale: string, firstDayOfWeek?: DayOfWeek): CalendarDateTime;
+export function endOfWeek(date: CalendarDate, locale: string, firstDayOfWeek?: DayOfWeek): CalendarDate;
+export function endOfWeek(date: DateValue, locale: string, firstDayOfWeek?: DayOfWeek): DateValue;
+export function endOfWeek(date: DateValue, locale: string, firstDayOfWeek?: DayOfWeek): DateValue {
+  return startOfWeek(date, locale, firstDayOfWeek).add({days: 6});
 }
 
 const cachedRegions = new Map<string, string>();
@@ -233,9 +246,9 @@ function getWeekStart(locale: string): number {
 }
 
 /** Returns the number of weeks in the given month and locale. */
-export function getWeeksInMonth(date: DateValue, locale: string): number {
+export function getWeeksInMonth(date: DateValue, locale: string, firstDayOfWeek?: DayOfWeek): number {
   let days = date.calendar.getDaysInMonth(date);
-  return Math.ceil((getDayOfWeek(startOfMonth(date), locale) + days) / 7);
+  return Math.ceil((getDayOfWeek(startOfMonth(date), locale, firstDayOfWeek) + days) / 7);
 }
 
 /** Returns the lesser of the two provider dates. */

packages/@internationalized/date/tests/queries.test.js

@@ -246,6 +246,13 @@ describe('queries', function () {
     it('should return the day of week in fr', function () {
       expect(getDayOfWeek(new CalendarDate(2021, 8, 4), 'fr')).toBe(2);
     });
+
+    it('should return the day of the week with a custom firstDayOfWeek', function () {
+      expect(getDayOfWeek(new CalendarDate(2021, 8, 4), 'en-US', 'mon')).toBe(2);
+      expect(getDayOfWeek(new CalendarDate(2021, 8, 4), 'en-US', 'tue')).toBe(1);
+      expect(getDayOfWeek(new CalendarDate(2021, 8, 4), 'fr-FR', 'mon')).toBe(2);
+      expect(getDayOfWeek(new CalendarDate(2021, 8, 4), 'fr-FR', 'tue')).toBe(1);
+    });
   });
 
   describe('startOfWeek', function () {
@@ -256,16 +263,28 @@ describe('queries', function () {
     it('should return the start of week in fr-FR', function () {
       expect(startOfWeek(new CalendarDate(2021, 8, 4), 'fr-FR')).toEqual(new CalendarDate(2021, 8, 2));
     });
+
+    it('should return the start of the week with a custom firstDayOfWeek', function () {
+      expect(startOfWeek(new CalendarDate(2021, 8, 4), 'en-US', 'mon')).toEqual(new CalendarDate(2021, 8, 2));
+      expect(startOfWeek(new CalendarDate(2021, 8, 4), 'en-US', 'tue')).toEqual(new CalendarDate(2021, 8, 3));
+      expect(startOfWeek(new CalendarDate(2021, 8, 4), 'fr-FR', 'sun')).toEqual(new CalendarDate(2021, 8, 1));
+    });
   });
 
   describe('endOfWeek', function () {
-    it('should return the start of week in en-US', function () {
+    it('should return the end of week in en-US', function () {
       expect(endOfWeek(new CalendarDate(2021, 8, 4), 'en-US')).toEqual(new CalendarDate(2021, 8, 7));
     });
 
-    it('should return the start of week in fr-FR', function () {
+    it('should return the end of week in fr-FR', function () {
       expect(endOfWeek(new CalendarDate(2021, 8, 4), 'fr-FR')).toEqual(new CalendarDate(2021, 8, 8));
     });
+
+    it('should return the end of the week with a custom firstDayOfWeek', function () {
+      expect(endOfWeek(new CalendarDate(2021, 8, 4), 'en-US', 'mon')).toEqual(new CalendarDate(2021, 8, 8));
+      expect(endOfWeek(new CalendarDate(2021, 8, 4), 'en-US', 'tue')).toEqual(new CalendarDate(2021, 8, 9));
+      expect(endOfWeek(new CalendarDate(2021, 8, 4), 'fr-FR', 'sun')).toEqual(new CalendarDate(2021, 8, 7));
+    });
   });
 
   describe('getWeeksInMonth', function () {
@@ -280,6 +299,13 @@ describe('queries', function () {
     it('should work for other calendars', function () {
       expect(getWeeksInMonth(new CalendarDate(new EthiopicCalendar(), 2013, 13, 4), 'en-US')).toBe(1);
     });
+
+    it('should support custom firstDayOfWeek', function () {
+      expect(getWeeksInMonth(new CalendarDate(2021, 8, 4), 'en-US', 'sun')).toBe(5);
+      expect(getWeeksInMonth(new CalendarDate(2021, 8, 4), 'en-US', 'mon')).toBe(6);
+      expect(getWeeksInMonth(new CalendarDate(2021, 10, 4), 'en-US', 'sun')).toBe(6);
+      expect(getWeeksInMonth(new CalendarDate(2021, 10, 4), 'en-US', 'mon')).toBe(5);
+    });
   });
 
   describe('getMinimumMonthInYear', function () {

packages/@react-aria/calendar/docs/useCalendar.mdx

@@ -129,7 +129,7 @@ function Calendar(props) {
         <Button {...prevButtonProps}>&lt;</Button>
         <Button {...nextButtonProps}>&gt;</Button>
       </div>
-      <CalendarGrid state={state} />
+      <CalendarGrid state={state} firstDayOfWeek={props.firstDayOfWeek} />
     </div>
   );
 }
@@ -152,7 +152,7 @@ function CalendarGrid({state, ...props}) {
   let {gridProps, headerProps, weekDays} = useCalendarGrid(props, state);
 
   // Get the number of weeks in the month so we can render the proper number of rows.
-  let weeksInMonth = getWeeksInMonth(state.visibleRange.start, locale);
+  let weeksInMonth = getWeeksInMonth(state.visibleRange.start, locale, props.firstDayOfWeek);
 
   return (
     <table {...gridProps}>
@@ -458,6 +458,14 @@ The `isReadOnly` boolean prop makes the Calendar's value immutable. Unlike `isDi
 <Calendar aria-label="Event date" value={today(getLocalTimeZone())} isReadOnly />
 ```
 
+### Custom first day of week
+
+By default, the first day of the week is automatically set based on the current locale. This can be changed by setting the `firstDayOfWeek` prop to `'sun'`, `'mon'`, `'tue'`, `'wed'`, `'thu'`, `'fri'`, or `'sat'`.
+
+```tsx example
+<Calendar aria-label="Event date" value={today(getLocalTimeZone())} firstDayOfWeek="mon" />
+```
+
 ### Labeling
 
 An aria-label must be provided to the `Calendar` for accessibility. If it is labeled by a separate element, an `aria-labelledby` prop must be provided using the `id` of the labeling element instead.

packages/@react-aria/calendar/docs/useRangeCalendar.mdx

@@ -129,7 +129,7 @@ function RangeCalendar(props) {
         <Button {...prevButtonProps}>&lt;</Button>
         <Button {...nextButtonProps}>&gt;</Button>
       </div>
-      <CalendarGrid state={state} />
+      <CalendarGrid state={state} firstDayOfWeek={props.firstDayOfWeek} />
     </div>
   );
 }
@@ -152,7 +152,7 @@ function CalendarGrid({state, ...props}) {
   let {gridProps, headerProps, weekDays} = useCalendarGrid(props, state);
 
   // Get the number of weeks in the month so we can render the proper number of rows.
-  let weeksInMonth = getWeeksInMonth(state.visibleRange.start, locale);
+  let weeksInMonth = getWeeksInMonth(state.visibleRange.start, locale, props.firstDayOfWeek);
 
   return (
     <table {...gridProps}>
@@ -477,6 +477,14 @@ The `isReadOnly` boolean prop makes the RangeCalendar's value immutable. Unlike
 <RangeCalendar aria-label="Trip dates" value={{start: today(getLocalTimeZone()), end: today(getLocalTimeZone()).add({ weeks: 1 })}} isReadOnly />
 ```
 
+### Custom first day of week
+
+By default, the first day of the week is automatically set based on the current locale. This can be changed by setting the `firstDayOfWeek` prop to `'sun'`, `'mon'`, `'tue'`, `'wed'`, `'thu'`, `'fri'`, or `'sat'`.
+
+```tsx example
+<RangeCalendar aria-label="Trip dates" firstDayOfWeek="mon" />
+```
+
 ### Labeling
 
 An aria-label must be provided to the `RangeCalendar` for accessibility. If it is labeled by a separate element, an `aria-labelledby` prop must be provided using the `id` of the labeling element instead.

packages/@react-aria/calendar/src/useCalendarGrid.ts

@@ -36,7 +36,11 @@ export interface AriaCalendarGridProps {
    * e.g. single letter, abbreviation, or full day name.
    * @default "narrow"
    */
-  weekdayStyle?: 'narrow' | 'short' | 'long'
+  weekdayStyle?: 'narrow' | 'short' | 'long',
+  /**
+   * The day that starts the week.
+   */
+  firstDayOfWeek?: 'sun' | 'mon' | 'tue' | 'wed' | 'thu' | 'fri' | 'sat'
 }
 
 export interface CalendarGridAria {
@@ -56,7 +60,8 @@ export interface CalendarGridAria {
 export function useCalendarGrid(props: AriaCalendarGridProps, state: CalendarState | RangeCalendarState): CalendarGridAria {
   let {
     startDate = state.visibleRange.start,
-    endDate = state.visibleRange.end
+    endDate = state.visibleRange.end,
+    firstDayOfWeek
   } = props;
 
   let {direction} = useLocale();
@@ -137,13 +142,13 @@ export function useCalendarGrid(props: AriaCalendarGridProps, state: CalendarSta
   let dayFormatter = useDateFormatter({weekday: props.weekdayStyle || 'narrow', timeZone: state.timeZone});
   let {locale} = useLocale();
   let weekDays = useMemo(() => {
-    let weekStart = startOfWeek(today(state.timeZone), locale);
+    let weekStart = startOfWeek(today(state.timeZone), locale, firstDayOfWeek);
     return [...new Array(7).keys()].map((index) => {
       let date = weekStart.add({days: index});
       let dateDay = date.toDate(state.timeZone);
       return dayFormatter.format(dateDay);
     });
-  }, [locale, state.timeZone, dayFormatter]);
+  }, [locale, state.timeZone, dayFormatter, firstDayOfWeek]);
 
   return {
     gridProps: mergeProps(labelProps, {