How often does Friday the 13th occur?

See proof at


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.