I'm interested in how superstitions came to be. For example, it's considered bad luck to light three of anything, such as cigarettes, with the same match. It originated during World War I because if a soldier held a lighted match for too long, he would make a target at night for the enemy.
Among Christians (and Western civilization) Friday the 13th is considered bad luck. No one knows exactly why 13 is considered bad luck, but Friday was the day that Jesus was crucified.