Group by day of the week (using arithmetic)

We can use modular arithmetic to calculate the day of the week.
We happen to know that 20 May 1962 was a Sunday. We calculate the number of days from that day and take mod 7 value. This tells us the day of the week: 0 is Sunday, 1 is Monday...

Specific to Oracle
Dates may be subtracted. The answer is in units of days.

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