firstDay

The day that each week begins.

<div class='spec' markdown='1'> Integer, *default*: `0` (Sunday) </div>

The default value depends on the current locale.

The value must be a number that represents the day of the week.

Sunday=0, Monday=1, Tuesday=2, etc.

If weekNumberCalculation is set to 'ISO', this option defaults to 1 (Monday).