These docs are for an old release.
Info on upgrading to version 2/3
firstDay 1.3
The day that each week begins.
Integer, default: 0 (Sunday)
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).
In versions 1.1 through 1.2.1, this option was known as weekStart.