The number 7 (the number argument in the MOD function) represents the day of week (dow) that you want.
To solve this formula, Excel first subtracts the dow (7 in this case) from the date, then feeds the result into the MOD function as the number. MOD returns the remainder of dividing that number by 7, which is then subtracted from date.
In Excel, the concept of "weekday" or "day of week" is based on the idea that each day of of the week has a specific number. With default settings, the WEEKDAY function treats Sunday as the first day of the week (1), and Saturday as the last day (7...
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