The Christian perspective on Friday and 13 is the most relevant today, but it's only one part of the Friday the 13th tradition.
Some trace the infamy of the number 13 back to ancient Norse culture. In Norse mythology, the beloved hero Balder was killed at a banquet by the mischievous god Loki, who crashed the party of twelve, bringing the group to 13. This story, as well as the story of the Last Supper, led to one of the most entrenched 13-related beliefs: You should never sit down to a meal in a group of 13.
Another significant piece of the legend is a particularly bad Friday the 13th that occurred in the middle ages. On a Friday the 13th in 1306, King Philip of France arrested the revered Knights Templar and began torturing them, marking the occasion as a day of evil. Check out this site to learn more.
Both Friday and the number 13 were once closely associated with capital punishment. In British tradition, Friday was the conventional day for public hangings, and there were supposedly 13 steps leading up to the noose.
Ultimately, the complex folklore of Friday the 13th doesn't have much to do with people's fears today. The fear has much more to do with personal experience. People learn at a young age that Friday the 13th is supposed to be unlucky, for whatever reason, and then they look for evidence that the legend is true. The evidence isn't hard to come by, of course. If you get in a car wreck on one Friday the 13th, lose your wallet, or even spill your coffee, that day will probably stay with you. But if you think about it, bad things, big and small, happen all the time. If you're looking for bad luck on Friday the 13th, you'll probably find it.
For more information about Friday the 13th and other superstitions, check out the links on the next page.