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Friday the Thirteenth

The Gregorian calendar follows a pattern of leap years which repeats every 400 years. There are 4,800 months in 400 years, so the 13th of the month occurs 4,800 times in this interval. The number of times the 13th occurs on each weekday is given in the table below. As shown by Brown (1933), the thirteenth of the month is slightly more likely to be on a Friday than on any other day.

Day Number of 13s Fraction
Sunday 687 14.31%
Monday 685 14.27%
Tuesday 685 14.27%
Wednesday 687 14.31%
Thursday 684 14.25%
Friday 688 14.33%
Saturday 684 14.25%

See also 13, Weekday


Ball, W. W. R. and Coxeter, H. S. M. Mathematical Recreations and Essays, 13th ed. New York: Dover, p. 27, 1987.

Brown, B. H. ``Solution to Problem E36.'' Amer. Math. Monthly 40, 607, 1933.

Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; and Vetterling, W. T. Numerical Recipes in FORTRAN: The Art of Scientific Computing, 2nd ed. Cambridge, England: Cambridge University Press, pp. 14-15, 1992.

© 1996-9 Eric W. Weisstein