firstDay

The day that each week begins.

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

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

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

This option is useful for UK users who need the week to start on Monday (1).

<div class='version-info' markdown='1'> In versions 1.1 through 1.2.1, this option was known as *weekStart*. </div>