Quick way to use modulus (remainder) arithmetic to quickly find the day of the week of any date in the year. Take Xmas. Today is Tue 6/11. (30-11) mod 7 = 5
+ 31 mod 7 = 1
+ 31 mod 7 = 4
+ 30 mod 7 = 6
+ 31 mod 7 = 2
+ 30 mod 7 = 4
+ 25 mod 7 = 1
1 means Xmas is one weekday past today, ie Wednesday
