So, is three a lot? According to Paul Lutus at Arachnoid:
- The probability that there will be a Friday the 13th during any given month is equal to the reciprocal of the number of weekdays: 1/7 or 14.1%.
- The average number of Friday the 13ths in a year is equal to the number of months divided by the number of weekdays: 12/7 or 1.71.
It turns out that this is not quite true. It was shown by Brown (I don't know his first name) in 1933 that the Gregorian Calendar (which we use) repeats itself exactly, every 400 years. In that time, there are 4800 months and 4800 13ths. Of those 4800 13ths, 688 occur on Friday. So the probability of a Friday the 13th is 688/4800 which is .143333..., which is slightly greater than 1/7. In fact, Friday is the most likely 13th, slightly. Of the 4800 13ths, Sunday is the 13th 687 times, Monday 685, Tuesday 685, Wednesday 687, Thursday 684, Friday 688, and Saturday 684.