Back in the 1970s, when I was writing and selling a system that was programmed in the language BCPL (a fore-runner of C), I gave a couple of courses in the language, and the exercise that I gave them at the end was to write a program to analyse the frequencies with with each date in the month fell on each day of the week. It turns out that in the present Gregorian calendar (which repeats its pattern every 4,000 years) the 13th day of the month falls slightly more often on a Friday than on the other days of the week, and that (as I recall) this is the largest deviation from uniformity.