Today is Friday the 13th, so I counted them. The Gregorian calendar repeats exactly every 400 years, because 400 years comes to 146,097 days, which is a whole number of weeks. In one full turn there are 4,800 thirteenths, and they don’t spread evenly across the week. Friday gets 688 of them, more than any other day. Thursday and Saturday get the fewest, 684 each. The gap is four days in four centuries, a side effect of where the leap years fall, too small for anyone to notice in a lifetime and perfectly real. So the date people are most wary of is also, by the thinnest margin the calendar allows, the one they get the most practice at.