Did you know that the day of the week for each day of the year is repeated every 400 years. So that, for instance, as today May 13th is a Friday, it means that May 13th 2416 is also on a Friday.
It is more likely that the 13th of a month will fall on a Friday that any other day of the week. In the 400 year cycle, that happens 688 times, but for the other days of the week between 684 and 687 times.