diff --git a/pkgs/intl4x/CHANGELOG.md b/pkgs/intl4x/CHANGELOG.md
index 0600a786..092b6279 100644
--- a/pkgs/intl4x/CHANGELOG.md
+++ b/pkgs/intl4x/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 1.0.0-alpha.3-wip
+
+- Add `calendar.dart` entrypoint exposing `Calendar` and `Weekday` with `Weekday.firstDayOfWeek` API.
+
 ## 1.0.0-alpha.2
 
 - Add `locale.dart` entrypoint exposing only `Locale`.
diff --git a/pkgs/intl4x/lib/calendar.dart b/pkgs/intl4x/lib/calendar.dart
new file mode 100644
index 00000000..fe3e806f
--- /dev/null
+++ b/pkgs/intl4x/lib/calendar.dart
@@ -0,0 +1,13 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// Calendar types and weekday information.
+///
+/// Provides [Calendar] and [Weekday] enums for working with calendar systems
+/// and week-start information.
+/// @docImport 'src/options.dart';
+library;
+
+export 'src/locale/locale.dart' show Locale;
+export 'src/options.dart' show Calendar, Weekday;
diff --git a/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week.dart b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week.dart
new file mode 100644
index 00000000..2dd7e21f
--- /dev/null
+++ b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week.dart
@@ -0,0 +1,15 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import '../ecma/ecma_native.dart'
+    if (dart.library.js_interop) '../ecma/ecma_web.dart';
+import '../locale/locale.dart';
+import '../options.dart';
+import 'first_day_of_week_stub.dart'
+    if (dart.library.js_interop) 'first_day_of_week_ecma.dart';
+import 'first_day_of_week_stub_4x.dart'
+    if (dart.library.ffi) 'first_day_of_week_4x.dart';
+
+Weekday getFirstDayOfWeek(Locale locale) =>
+    useBrowser ? getFirstDayOfWeekECMA(locale) : getFirstDayOfWeek4X(locale);
diff --git a/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_4x.dart b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_4x.dart
new file mode 100644
index 00000000..146e60ce
--- /dev/null
+++ b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_4x.dart
@@ -0,0 +1,22 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:icu4x/icu4x.dart' as icu;
+
+import '../locale/locale.dart';
+import '../locale/locale_4x.dart';
+import '../options.dart';
+
+Weekday getFirstDayOfWeek4X(Locale locale) {
+  final weekInfo = icu.WeekInformation((locale as Locale4x).get4X);
+  return switch (weekInfo.firstWeekday) {
+    icu.Weekday.monday => Weekday.monday,
+    icu.Weekday.tuesday => Weekday.tuesday,
+    icu.Weekday.wednesday => Weekday.wednesday,
+    icu.Weekday.thursday => Weekday.thursday,
+    icu.Weekday.friday => Weekday.friday,
+    icu.Weekday.saturday => Weekday.saturday,
+    icu.Weekday.sunday => Weekday.sunday,
+  };
+}
diff --git a/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_ecma.dart b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_ecma.dart
new file mode 100644
index 00000000..2af3eb40
--- /dev/null
+++ b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_ecma.dart
@@ -0,0 +1,36 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:js_interop';
+import 'dart:js_interop_unsafe';
+
+import '../locale/locale.dart';
+import '../options.dart';
+
+@JS('Intl.Locale')
+extension type _LocaleJS._(JSObject _) implements JSObject {
+  external factory _LocaleJS(String s);
+  external JSObject? get weekInfo;
+  external JSObject? getWeekInfo();
+}
+
+Weekday getFirstDayOfWeekECMA(Locale locale) {
+  final jsLocale = _LocaleJS(locale.toLanguageTag());
+  JSObject? weekInfo;
+  if (jsLocale.hasProperty('getWeekInfo'.toJS).toDart) {
+    weekInfo = jsLocale.getWeekInfo();
+  } else if (jsLocale.hasProperty('weekInfo'.toJS).toDart) {
+    weekInfo = jsLocale.weekInfo;
+  }
+  if (weekInfo != null) {
+    final firstDayJS = weekInfo.getProperty('firstDay'.toJS);
+    if (firstDayJS != null && firstDayJS.isA<JSNumber>()) {
+      final firstDay = (firstDayJS as JSNumber).toDartInt;
+      if (firstDay >= 1 && firstDay <= 7) {
+        return Weekday.fromIsoIndex(firstDay);
+      }
+    }
+  }
+  return Weekday.monday;
+}
diff --git a/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_stub.dart b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_stub.dart
new file mode 100644
index 00000000..6658a3e1
--- /dev/null
+++ b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_stub.dart
@@ -0,0 +1,9 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import '../locale/locale.dart';
+import '../options.dart';
+
+Weekday getFirstDayOfWeekECMA(Locale locale) =>
+    throw UnimplementedError('Cannot use ECMA outside of web environments.');
diff --git a/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_stub_4x.dart b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_stub_4x.dart
new file mode 100644
index 00000000..21d5d236
--- /dev/null
+++ b/pkgs/intl4x/lib/src/first_day_of_week/first_day_of_week_stub_4x.dart
@@ -0,0 +1,9 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import '../locale/locale.dart';
+import '../options.dart';
+
+Weekday getFirstDayOfWeek4X(Locale locale) =>
+    throw UnimplementedError('Cannot use ICU4X in web environments.');
diff --git a/pkgs/intl4x/lib/src/options.dart b/pkgs/intl4x/lib/src/options.dart
index 6a70a01b..705cbd1a 100644
--- a/pkgs/intl4x/lib/src/options.dart
+++ b/pkgs/intl4x/lib/src/options.dart
@@ -2,8 +2,45 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-/// Common options used across intl4x libraries.
-library;
+import 'first_day_of_week/first_day_of_week.dart';
+import 'locale/locale.dart' show Locale;
+
+/// Weekdays.
+enum Weekday {
+  monday(1),
+  tuesday(2),
+  wednesday(3),
+  thursday(4),
+  friday(5),
+  saturday(6),
+  sunday(7);
+
+  /// The ISO 8601 standard index, where Monday is 1 and Sunday is 7.
+  final int isoIndex;
+
+  const Weekday(this.isoIndex);
+
+  /// Returns the first day of the week for the given [locale].
+  factory Weekday.firstDayOfWeek([Locale? locale]) =>
+      getFirstDayOfWeek(locale ?? Locale.system);
+
+  /// Returns the [Weekday] corresponding to the given ISO 8601 index (1 for
+  /// Monday, 7 for Sunday).
+  factory Weekday.fromIsoIndex(int isoIndex) => switch (isoIndex) {
+    1 => Weekday.monday,
+    2 => Weekday.tuesday,
+    3 => Weekday.wednesday,
+    4 => Weekday.thursday,
+    5 => Weekday.friday,
+    6 => Weekday.saturday,
+    7 => Weekday.sunday,
+    _ => throw ArgumentError.value(
+      isoIndex,
+      'isoIndex',
+      'Must be between 1 and 7',
+    ),
+  };
+}
 
 /// Calendar types for date and time formatting.
 enum Calendar {
diff --git a/pkgs/intl4x/pubspec.yaml b/pkgs/intl4x/pubspec.yaml
index ba500c25..7e62605d 100644
--- a/pkgs/intl4x/pubspec.yaml
+++ b/pkgs/intl4x/pubspec.yaml
@@ -1,7 +1,7 @@
 name: intl4x
 description: >-
   A lightweight modular library for internationalization (i18n) functionality.
-version: 1.0.0-alpha.2
+version: 1.0.0-alpha.3-wip
 repository: https://github.com/dart-lang/i18n/tree/main/pkgs/intl4x
 issue_tracker: https://github.com/dart-lang/i18n/issues?q=is%3Aissue+is%3Aopen+label%3Apackage%3Aintl4x
 
diff --git a/pkgs/intl4x/test/calendar_test.dart b/pkgs/intl4x/test/calendar_test.dart
new file mode 100644
index 00000000..1130e3ed
--- /dev/null
+++ b/pkgs/intl4x/test/calendar_test.dart
@@ -0,0 +1,46 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:intl4x/calendar.dart';
+import 'package:test/test.dart';
+
+import 'utils.dart';
+
+void main() {
+  group('firstDayOfWeek', () {
+    testWithFormatting('en-US', () {
+      final locale = Locale.parse('en-US');
+      expect(Weekday.firstDayOfWeek(locale), Weekday.sunday);
+    });
+
+    testWithFormatting('fr-FR', () {
+      final locale = Locale.parse('fr-FR');
+      expect(Weekday.firstDayOfWeek(locale), Weekday.monday);
+    });
+
+    testWithFormatting('ar-EG', () {
+      final locale = Locale.parse('ar-EG');
+      expect(Weekday.firstDayOfWeek(locale), Weekday.saturday);
+    });
+  });
+
+  group('Weekday ISO enum', () {
+    test('values', () {
+      expect(Weekday.monday.isoIndex, 1);
+      expect(Weekday.tuesday.isoIndex, 2);
+      expect(Weekday.wednesday.isoIndex, 3);
+      expect(Weekday.thursday.isoIndex, 4);
+      expect(Weekday.friday.isoIndex, 5);
+      expect(Weekday.saturday.isoIndex, 6);
+      expect(Weekday.sunday.isoIndex, 7);
+    });
+
+    test('fromIsoIndex', () {
+      expect(Weekday.fromIsoIndex(1), Weekday.monday);
+      expect(Weekday.fromIsoIndex(7), Weekday.sunday);
+      expect(() => Weekday.fromIsoIndex(0), throwsArgumentError);
+      expect(() => Weekday.fromIsoIndex(8), throwsArgumentError);
+    });
+  });
+}
diff --git a/pkgs/intl4x/test/locale_test.dart b/pkgs/intl4x/test/locale_test.dart
new file mode 100644
index 00000000..6c7bb76d
--- /dev/null
+++ b/pkgs/intl4x/test/locale_test.dart
@@ -0,0 +1,40 @@
+// Copyright (c) 2026, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:intl4x/datetime_format.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Locale', () {
+    test('parse and toLanguageTag', () {
+      final locale = Locale.parse('en-US');
+      expect(locale.toLanguageTag(), 'en-US');
+      expect(locale.toString(), 'en-US');
+    });
+
+    test('toLanguageTag', () {
+      final locale = Locale.parse('de-DE');
+      expect(locale.toLanguageTag(), 'de-DE');
+    });
+
+    test('withCalendar', () {
+      final locale = Locale.parse('en-US').withCalendar(Calendar.buddhist);
+      expect(locale.toLanguageTag(), 'en-US-u-ca-buddhist');
+    });
+
+    test('withNumberingSystem', () {
+      final locale = Locale.parse(
+        'en-US',
+      ).withNumberingSystem(NumberingSystem.arabic);
+      expect(locale.toLanguageTag(), 'en-US-u-nu-arab');
+    });
+
+    test('withClockStyle', () {
+      final locale = Locale.parse(
+        'en-US',
+      ).withClockStyle(ClockStyle.oneToTwelve);
+      expect(locale.toLanguageTag(), 'en-US-u-hc-h12');
+    });
+  });
+}