Today is Friday the 13th. It's the first of 3 this year. Is 3 more than you'd expect in a year? Here's how I'd calculate it. 12 months, each with a 13th. 7 days in a week, so each month has 1/7 chance of having a Friday 13. 1/7 x 12 = 1.71. So this year is significantly more unlucky than others, I guess. OK, if I got this trivial math problem wrong, I'm really embarrassed, but so what. A more complex question would be whether or not numerically days of the week are evenly distributed, as I have assumed in my calculations. Also, does the occurrence of a Friday 13 effect the probability of another in the year? Interesting number theory questions with hints of chaos theory as well.