Determination of the day of the week - Wikipedia
https://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week
Concepts. In numerical calculation, the days of the week are represented as weekday numbers. If Monday is the first day of the week, the days may be coded 1 to 7, for Monday through Sunday, as is practiced in ISO 8601.The day designated with 7 may also be counted as 0, by applying the arithmetic modulo 7, which calculates the remainder of a number after division by 7.
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