Last Friday was a Friday the 13th. I read in the paper the other day that the 13th day of the month was more likely to fall on a Friday than any other month. I was a bit skeptical about this, but having checked it out, it is true, but only when you take into account that unusual leap year rule that says years divisible by 100 (but not 400) are not leap years.

This once in a 100 years aberration to the pattern of the calendar skews the distribution of weekdays such that over a 400 year period you get the 13th day of a month falling most often on a Friday (688 times), but least often on a Thursday (684 times.)

So the probabilities for the weekday of the 13th of the month comes out as follows …

Sunday: 14.3125%
Monday: 14.27083%
Tuesday: 14.27083%
Wednesday: 14.3125%
Thursday: 14.25%
Friday: 14.33333%
Saturday: 14.25%

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