Happy Friday the 13th! The next one will be in October. Interestingly, as Dr. Math explains, the 13th is slightly more likely to fall on a Friday than on any other day.

It turns out that every year has at least one Friday the 13th. The largest possible number of Friday the 13ths in one year is three. Surprisingly, it turns out that 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.

Wolfram.com provides more mathematical details about Friday the 13th. Any given 13th of a month is 14.33% likely to land on a Friday. (If the odds were equal for every day, one-seventh is about 0.1429.) The simple explanation for this unintuitive result is that leap years cause some strange patterns, and it's pure chance that we happened to start using them in a way that favored Friday the 13th over Xday the 13th.

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