Friday the 13th: The Geek Approach

"Friday the Thirteenth "
Wolfram Research / scienceworld.Wolfram.com

"The Gregorian calendar follows a pattern of leap years which repeats every 400 years. This is true since the number of days in 400 Gregorian years is

which is an exact number of weeks since it is divisible by 7 (namely, ). There are 4,800 months in 400 years, so ...."

Well, you get the idea.

As a sort of bonus, there's a snippet of Mathematica code that can generate counts for how many times a given weekday occurs.