The superstitious association of the number thirteen with bad luck came from an earlier belief that if thirteen people ate together at a table, one would die within a year. The root of this was of course the Last Supper, where Jesus and his twelve disciples were the thirteen diners, and the Crucifixion that followed that night is the bad luck. Since Good Friday is the calendar marker for the Crucifixion, I guess someone just tacked the Friday on there and that compounded with 13 led masses of people to believe that Friday the 13th was an all-time low for good luck. Now there are 13 clubs in some large cities that make a sort of game of superstition. Every table has thirteen diners who break mirrors before eating, have funeral dirges played, open umbrellas, and probably do all sorts of other things.

A mathematician whose name I can't recall stated that if fifteen people of random ages and lifestyles were chosen to eat together, the odds were approximately one to one that one would die in a year. I don't see why they even have to eat together, but I'm not a superstition mathematician either.