Proof about Friday the 13th

Question

Few years ago I learned that there are more friday the 13th in the 400 years period of our calendar, than all other days, but I have never found any proof !

Can you help me please ?

Answer 1

In the Gregorian calendar, the relation between dates and weekdays repeats exactly every 400 years.

Over each 400-year period, the 13ths of the 4800 months fall on the following weekdays:

Mon: 685 times
Tue: 685 times
Wed: 687 times
Thu: 684 times
Fri: 688 times
Sat: 684 times
Sun: 687 times

(which I found simply by letting a computer iterate through all 4800 months, keeping count). So Friday is indeed slightly more common than other days of the week among all 13ths.