How often does Friday the 13th occur?

See proof at http://wiki.answers.com/Q/Does_every_year_contain_at_least_one_Friday_the_13th

Addendum:

If you count the occurrences of each number in mod 7, you will know how many times Friday the 13th will occur at most or minimum. 3 comes up 3x, but let's not forget leap year. We'd get 0 3x, but to answer the question, it can happen at most 3x.

Let's see about a minimum... 1, 2, & 4 all come up just once, so 1x minimum.

There you have it: at most 3x, at least once.