fix: iso8601 week start and use intl.locale when possible for week start

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

@@ -223,6 +223,7 @@ export function endOfWeek(date: DateValue, locale: string, firstDayOfWeek?: DayO
 }
 
 const cachedRegions = new Map<string, string>();
+const cachedWeekInfo = new Map<string, {firstDay: number}>();
 
 function getRegion(locale: string): string | undefined {
   // If the Intl.Locale API is available, use it to get the region for the locale.
@@ -251,8 +252,47 @@ function getRegion(locale: string): string | undefined {
 function getWeekStart(locale: string): number {
   // TODO: use Intl.Locale for this once browsers support the weekInfo property
   // https://github.com/tc39/proposal-intl-locale-info
-  let region = getRegion(locale);
-  return region ? weekStartData[region] || 0 : 0;
+  let weekInfo = cachedWeekInfo.get(locale);
+  if (!weekInfo) {
+    if (Intl.Locale) {
+      // @ts-ignore
+      let localeInst = new Intl.Locale(locale);
+      if ('getWeekInfo' in localeInst) {
+        // @ts-expect-error
+        weekInfo = localeInst.getWeekInfo();
+        if (weekInfo) {
+          cachedWeekInfo.set(locale, weekInfo);
+          return weekInfo.firstDay;
+        }
+      }
+    }
+    let region = getRegion(locale);
+    if (locale.includes('-fw-')) {
+      let day = locale.split('-fw-')[1];
+      if (day === 'mon') {
+        weekInfo = {firstDay: 1};
+      } else if (day === 'tue') {
+        weekInfo = {firstDay: 2};
+      } else if (day === 'wed') {
+        weekInfo = {firstDay: 3};
+      } else if (day === 'thu') {
+        weekInfo = {firstDay: 4};
+      } else if (day === 'fri') {
+        weekInfo = {firstDay: 5};
+      } else if (day === 'sat') {
+        weekInfo = {firstDay: 6};
+      } else {
+        weekInfo = {firstDay: 0};
+      }
+    } else if (locale.includes('u-ca-iso8601')) {
+      weekInfo = {firstDay: 1};
+    } else {
+      weekInfo = {firstDay: region ? weekStartData[region] || 0 : 0};
+    }
+    cachedWeekInfo.set(locale, weekInfo);
+  }
+
+  return weekInfo.firstDay;
 }
 
 /** Returns the number of weeks in the given month and locale. */

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

@@ -273,6 +273,15 @@ describe('queries', function () {
       expect(startOfWeek(new CalendarDate(2021, 8, 4), 'fr-FR', 'sun')).toEqual(new CalendarDate(2021, 8, 1));
       expect(startOfWeek(new CalendarDate(2021, 8, 4), 'en-US', 'thu')).toEqual(new CalendarDate(2021, 7, 29));
     });
+
+    it('should return the start of the week in en-US-u-ca-iso8601', function () {
+      // start of week is monday
+      expect(startOfWeek(new CalendarDate(2021, 8, 4), 'en-US-u-ca-iso8601')).toEqual(new CalendarDate(2021, 8, 2));
+      expect(startOfWeek(new CalendarDate(2021, 8, 4), 'fr-FR-u-ca-iso8601')).toEqual(new CalendarDate(2021, 8, 2));
+
+      // override first day of week
+      expect(startOfWeek(new CalendarDate(2021, 8, 4), 'en-US-u-ca-iso8601-fw-tue')).toEqual(new CalendarDate(2021, 8, 3));
+    });
   });
 
   describe('endOfWeek', function () {

packages/react-aria-components/stories/Calendar.stories.tsx

@@ -10,7 +10,18 @@
  * governing permissions and limitations under the License.
  */
 
-import {Button, Calendar, CalendarCell, CalendarGrid, CalendarStateContext, Heading, RangeCalendar} from 'react-aria-components';
+import {
+  Button,
+  Calendar,
+  CalendarCell,
+  CalendarGrid,
+  CalendarProps,
+  CalendarStateContext,
+  DateValue,
+  Heading,
+  I18nProvider,
+  RangeCalendar
+} from 'react-aria-components';
 import {CalendarDate, parseDate} from '@internationalized/date';
 import {Meta, StoryObj} from '@storybook/react';
 import React, {useContext} from 'react';
@@ -120,6 +131,41 @@ export const CalendarMultiMonth: CalendarStory = {
   }
 };
 
+
+interface CalendarFirstDayOfWeekExampleProps extends CalendarProps<DateValue> {
+  locale: string
+}
+
+export const CalendarFirstDayOfWeekExample: StoryObj<CalendarFirstDayOfWeekExampleProps> = {
+  render: function Example(args) {
+    return (
+      <div>
+        <I18nProvider locale={args.locale}>
+          <Calendar style={{width: 220}}>
+            <div style={{display: 'flex', alignItems: 'center'}}>
+              <Button slot="previous">&lt;</Button>
+              <Heading style={{flex: 1, textAlign: 'center'}} />
+              <Button slot="next">&gt;</Button>
+            </div>
+            <CalendarGrid style={{width: '100%'}}>
+              {date => <CalendarCell date={date} style={({isSelected, isOutsideMonth}) => ({display: isOutsideMonth ? 'none' : '', textAlign: 'center', cursor: 'default', background: isSelected ? 'blue' : ''})} />}
+            </CalendarGrid>
+          </Calendar>
+        </I18nProvider>
+      </div>
+    );
+  },
+  args: {
+    locale: 'en-US-u-ca-iso8601-fw-tue'
+  },
+  argTypes: {
+    locale: {
+      control: 'select',
+      options: ['en-US-u-ca-iso8601-fw-tue', 'en-US-u-ca-iso8601', 'en-US', 'fr-FR-u-ca-iso8601-fw-tue', 'fr-FR-u-ca-iso8601', 'fr-FR']
+    }
+  }
+};
+
 export const RangeCalendarExample: RangeCalendarStory = {
   render: () => (
     <RangeCalendar style={{width: 220}}>