How common are Friday the 13ths?
Every year has at least one Friday the 13th. The largest possible number of Friday the 13ths in one year is three. Interestingly, a 13th is slightly more likely to fall on a Friday than any other day. This happens because the leap year rules make some patterns of weekdays happen more often than others. The winning patterns include many Friday the 13ths. - faqkids.com; wikipedia.org; howstuffworks.com