Paraskevidekatriaphobia — fear of Friday the 13th
So where does it come from — the fear of 13? Its origins can be traced to Norse mythology and a dinner party at Valhalla, home of the god Odin, where Odin and 11 of his closest god-friends were gathered one night to party. Everyone was having fun, but then Loki, the dastardly god of evil and turmoil, showed up uninvited, making it a crowd of 13. The beloved god Balder tried to boot Loki out of the house, the legend goes, and in the scuffle that followed he suffered a deathblow from a spear of mistletoe.
From that mythological start, the number 13 has plowed a path of devastation through history. There were 13 people at Christ’s Last Supper, including the double-crossing Judas Iscariot. The ill-fated Apollo 13 lunar mission left the launching pad at 13:13 hours and was aborted on April 13. Friday hasn’t been much kinder to us. Friday was execution day in ancient Rome — Jesus was crucified on a Friday. Put it all together, and Friday the 13th spells trouble for triskaidekaphobics. It’s a testament to the phobia’s prevalence that Hollywood was able to parlay our fear into a hugely successful series of slasher movies starring a hockey-masked guy named Jason.
But triskaidekaphobia isn’t an exclusively American affliction. Italians omit the number 13 from their national lottery. There is a hush-hush organization in France whose exclusive purpose is to provide last-minute guests for dinner parties, so that no party host ever has to suffer the curse of entertaining 13 guests.
Excerpted from Jon Bowen, writing at Slate.
About.com has five pages of background on the superstition.
And Urban Legends has a lengthy page.