Ignore the goofy url, this is a really fascinating mental exercise. I just started reading it, but i'm definitely going to try to commit it to memory:
http://www.h2limousine.com/calendar2.php
Basically, the day of the week (1=sunday, 2= Monday...0=saturday) is:
(Month code + year code + day of the month) mod 7.
Month codes:
January: 1
February: 4
March: 4
April: 0
May: 2
June: 5
July: 0
August: 3
September: 6
October: 1
November: 4
December: 6
Year codes:
2005: 5
2006: 6
2007: 0
2008: 2
2009: 3
2010: 4
The 'mod' operation means 'subtract seven until you can't subtract seven any more without getting a negative number. Thus, Mod 25 would be [
25-7 (18) - 7 (11) - 7 ]
=4
A practical example. My sister's birthday is April 15th. Her 21st is this year, and I'm curious whether it's on a weekend, as she wants me to come out.
Month: 0
Year: 2
Day: 15
17 mod 7 = 3. d'oh, it's a Tuesday.
That's the cliff notes version, but check out the site if it at all interests you
Anyone else have any cool math tricks? I class this about three times as easy as memorizing a substancial number of digits of pi (which I still have out to 3.141592653589793238...used to have more), and thirty times as useful
