Prove that in each year, the 13th day of some month occurs on a Friday Prove that in each year, the 13th day of some month occurs on a  Friday.
No clue... please help!

Figure out where the 13th day of each month falls relative to January 1st (remember there are two possibilities, corresponding to leap year or non leap year). There are only seven possible values: 0, 1, 2, 3, 4, 5, 6 days after the day on which Jan. 1 falls. If each of those values occurs at least once, then at least one of those days must be a Friday. Can you take it from there?
