The Gregorian calendar follows a pattern of leap years which repeats every 400 years. This is true since the number of days in 400 Gregorian years is

which is an exact number of weeks since it is divisible by 7 (namely, ). 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.