[DatePicker] Change the behavior of firstDayOfWeek to follow ISO

[oliviertassinari]

I'm moving the conversion here.

Change the default value of firstDayOfWeek from 0 to 1

@oliviertassinari Shouldn't by standard Monday be 0? @mbrookes I'm no expert on the standard. Is defaulting to 1 (yet starting from Saturday) against the standard?

@alitaheri I'm not sure to follow your point. We have 0=Sunday and 1=Monday. Do you want to change it to 0=Monday?

What I mean is, if by standard the start of the week is Monday, then it's more convenient to start COUNTING from Monday too. in other words it will be like: 6=Sunday and 0=Monday and so on. with default still being 0. instead of changing the default to 1 we rotate the days the numbers correspond to. So the standard is respected in every aspect.

I thought the same, which is why I raised the question of the default in the PR, since changing the numbering in a future release could have more impact than just changing the default.
However, Date.prototype.getDay() has Sunday=0, so at least it's consistent with that.

Let's see what moment is doing. you have:

var moment = require("moment")
moment.locale('fr');

console.log(moment().isoWeekday(0).format("dddd")); // Sunday
console.log('');
console.log(moment().weekday(0).format("dddd")); // (locale aware) Monday

(https://tonicdev.com/569e2ecaabe29a0c00e1727c/569e314e3df26e0d00fc271d)

Looks like 0 is the iso weekday for Sunday and that we shouldn't change anything 😁.

[mbrookes]

Related to #1839.

[mbrookes]

Why is this closed? Do we not still want firstDayOfWeek to follow ISO (Monday)?

It just means the default is 1, rather than 0. I think the conversation got derailed slightly by discussing what 0 should be, but that is agreed - 0 = Sunday.

What should the default be?

[oliviertassinari]

@mbrookes You are right, my bad. I think that firstDayOfWeek default value should be 1 = Monday.

[mbrookes]

I have a hacky fix for #1839. I'll submit a PR for them both later.

[iheidt]

The default for firstDayOfWeek should be 0 = Sunday.

Look at Google examples:
https://material.io/guidelines/components/pickers.html#pickers-date-pickers