Triskaidekaphobia is an irrational fear of the number thirteen. And today is the only "Friday the 13th" of 2010! So it must be an especially scary day for triskaidekaphobes. Enjoy the fear, my friends! Mwah ha ha ha ha!
From an article on Friday the 13th that appears at the National Geographic website comes this explanation of the phobia:
"Friday the 13th superstitions are rooted in ancient bad-luck associations with the number 13 and the day Friday, said Donald Dossey, a folklore historian and author of Holiday Folklore, Phobias and Fun.
"The two unlucky entities ultimately combined to make one super unlucky day.
"Dossey traces the fear of the number 13—aka, triskaidekaphobia—to a Norse myth about 12 gods having a dinner party at Valhalla, Norse mythology's heaven. In walked the uninvited 13th guest, the mischievous god Loki. Once there, Loki arranged for Hoder, the blind god of darkness, to shoot Balder the Beautiful, the god of joy and gladness, with a mistletoe-tipped arrow.
"'Balder died, and the whole Earth got dark. The whole Earth mourned. It was a bad, unlucky day,' Dossey said."There is also a biblical reference to the unlucky number 13. Judas, the apostle said to have betrayed Jesus, was the 13th guest to the Last Supper."
Read more at the link above.
According to howstuffworks.com, the fear of Friday the 13th specifically is known as paraskevidekatriaphobia.
Say that thirteen times fast!