Fun fact: while a much more often occurrence than once in a lifetime, “Thursday the 20th” is tied with “Saturday the 20th” as the least-likely combination of days of the week with the 20th day of the month, even though you’d think the chances would be exactly 1/7.

Here’s the math about the Gregorian calendar that explains why. (Even though the post is about Friday the 13th, it straightforwardly can be applied to any other day/date combination as well.)