As there are 24 leap years in three out of four centuries and 25 in the fourth, in a 400 year period there are 303*365+97*366 = 146097 = 20871*7 days. So the weekday pattern repeats every 400 years, as this is exactly 20871 weeks. The relationship of weekdays to dates is such that the 13th of the month falls more often on Friday in the 400 year period than on any other day of the week - not a large bias, but one which doesn't go away! The 13th falls on Monday 685, Tuesday 685, Wednesday 687, Thursday 684, Friday 688, Saturday 684, and Sunday 687 times.
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