These docs are for an old release.
Info on upgrading to v6
firstDay
The day that each week begins.
Integer, default: 0 (Sunday)
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).