bug(datepicker): Luxon date adapter consider by default Sunday as the first day of week for any locale

[MikaStark]

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

No response

Description

When using the Luxon date adapter, the first day of the week is always set to Sunday, regardless of the selected locale. However, in certain locales (e.g., French), the first day of the week should be Monday. This behavior can cause inconsistencies in applications relying on localized date formats.

To address this issue, we can provide custom options for the Luxon date adapter by using a factory function. This factory dynamically calculates the correct firstDayOfWeek based on the current locale, leveraging Luxon’s Info.getStartOfWeek method.

Here's how to configure it:

import { MAT_LUXON_DATE_ADAPTER_OPTIONS } from '@angular/material-luxon-adapter';
import { MAT_DATE_LOCALE } from '@angular/material/core';
import { Info } from 'luxon';
import { inject } from '@angular/core';

{
  provide: MAT_LUXON_DATE_ADAPTER_OPTIONS,
  useFactory: () => {
    const locale = inject<string>(MAT_DATE_LOCALE); // Retrieve the current locale
    const firstDayOfWeek = Info.getStartOfWeek({ locale }); // Get the first day of the week for the locale
    return { firstDayOfWeek };
  },
},

Luxon relies on the Intl.Locale.prototype.getWeekInfo API to determine the first day of the week. Unfortunately, this API is not supported in Firefox (as of now). However, Luxon includes a fallback mechanism that assumes Monday as the first day of the week, which aligns with the ISO standard.

For more details on getWeekInfo, refer to the MDN documentation.

Reproduction

StackBlitz link: https://stackblitz.com/edit/gao3m1fm?file=src%2Fexample%2Fdatepicker-overview-example.ts,src%2Fmain.ts
Steps to reproduce:

1. Open date picker and see that "D" ("Dimanche" aka. Sunday) is the first day of the week which is invalid as LOCAL_ID is "fr"
2. Uncomment the last provider in bootstrapApplication
3. Open date picker and see that "L" ("Lundi" aka. Monday) is the correct first day

Expected Behavior

First day of week should depends on selected locale.

Actual Behavior

The first day of the week is always set to Sunday, regardless of the selected locale.

Environment

• Angular: 19.0.5
• CDK/Material: 19.0.4
• Browser(s): Chrome 131.0.6778.205 (64 bits)
• Operating System (e.g. Windows, macOS, Ubuntu): Windows 11

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