A Gentle Introduction to
How to work with dates; adding, subtracting and formatting.
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...