Hmmm on my machine this test keeps failing.

Yeah, because on my machine it's:

The "--day-id: 1;" will be "26" in my case

Arf! Indeed, I should have force a "fr-BE" or "en" culture on this test.
Do you want me to create a fix in +/-30min?

I guess i will just add FirstDayOfWeek="@FirstDayOfWeek" to SimpleMudDatePickerTest and set it

var comp = OpenPicker(parameters => parameters
                .Add(x => x.PickerMonth, new DateTime(2026, 2, 15))
                .Add(x => x.FirstDayOfWeek, DayOfWeek.Sunday));

instead of the culture approach

Oh yeah that's even cleaner! Sorry for introducing an issue.

I’m still looking into this. SimpleMudDatePickerTest forces the culture, which makes it hard for me to reason about this issue.

One idea would be to change the test to assert the rendered result instead of relying on culture. Rather than forcing a culture or FirstDayOfWeek, we could check that the picker renders the expected number of visible days for the month.

For example:

[Test]
public void GetMonthStart_Should_NormalizeToFirstDayOfMonth()
{
    var testDateTime = new DateTime(2026, 2, 15);

    var comp = OpenPicker(parameters => parameters
        .Add(x => x.PickerMonth, testDateTime));

    var visibleDaysCount = comp
        .FindAll(".mud-button-root.mud-icon-button.mud-ripple.mud-ripple-icon.mud-picker-calendar-day.mud-day:not(.mud-hidden)")
        .Count;

    visibleDaysCount.Should().Be(DateTime.DaysInMonth(testDateTime.Year, testDateTime.Month));
}

This way the test just checks that all days of the month are rendered, regardless of culture.

Yeah, that's also a good idea, but I already implemented the fix in other PR with the DayOfWeek.Sunday .
There is another bug related with the FirstDayOfWeek.
If you open the editable picker at 01/01/0001 when FirstDayOfWeek=DayOfWeek.Monday you will see "6" twice on Sunday and Saturday, but if you open DayOfWeek.Sunday then its ok. Apparently this is some ancient bug. If you want, you can look at it.