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How common are Friday the 13ths?

You can use a perpetual calendar to work out how many Friday the 13ths there are for every possible pattern of weekdays in a year. 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.

Here’s a table from Eric Weisstein’s World of Astronomy that shows how often the 13th falls on each day of the week, over four hundred years.

DayNumber of 13thsPercent

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