firstDay *1.3*
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The day that each week begins.

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

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In versions 1.1 through 1.2.1, this option was known as *weekStart*.
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