The fear of Friday the 13th is called paraskavedekatriaphobia. I know it sounds like I just made that up, but it's true. There are 4,800 months in 400 years, soooo the 13th of the month occurs 4,800 times during this interval. The thirteenth of the month is slightly more likely to be on a Friday than on any other day. On average, there are 1.72 "Friday the 13ths" per calendar year. Make a Wish.