The divide-by-four test for a leap year is modified if the year is divisible by 100 and again if divisible by 400, thus 1900 was not a leap year, 2000 was. The effect of this modification is that the number of days in 400 years is a multiple of seven. It is thus a simple task (for a computer) to examine the days in any 400 year period and observe that the 13th of the month is more likely to fall on a Friday than any other day. Here is the distribution of 13ths for a 400 year period, from 1800 to 5799:
Sun 6870
Mon 6850
Tue 6850
Wed 6870
Thu 6840
Fri 6880
Sat 6840
It’s not a huge effect, but the 13th lands on a Friday 23 more times than an even distribution would provide.

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