- In a Norse myth, 12 gods were having a party in Valhalla. The uninvited 13th guest, Loki, conspired at the party and was able to get one god to kill another one.
- Judas, the 13th apostle, betrayed Jesus.
- In Rome, witches gathered in groups of 12 with the 13th guest being the devil.
- 12 is the complete number, and 13 is one above complete.
- Friday was the day Jesus was crucified.
- Cain killed Abel on Friday the 13th.
My $0.02: The belief that this day is unlucky is just a product of people's imaginations. You must consider the fact that many of these superstitions arose during ancient times when people didn't understand the world as much as they do now. Of course most those people would believe supernatural dates had some sort of spooky affect on their lives! They didn't have any idea of what dictated the flow of their lives. Numbers would likely hold some power to them. And yes, you could point out that you are more likely to be admitted to the hospital on Friday the 13th, but this is just a result of the fact that people believe the day is unlucky, rather than proof that it is.