# firstDay

The day that each week begins.

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Integer, *default*: `0` (Sunday)

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The default value depends on the current [locale](locale.md).

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

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

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