triskaidekaphobia

triskaidekaphobia \tris-ky-dek-uh-FOH-bee-uh\, noun:
A morbid fear of the number 13 or the date Friday the 13th.

Triskaidekaphobia is a fairly new word (first found in print in 1911) formed from Greek treiskaideka, triskaideka, "thirteen" (treis, "three" + kai, "and" + deka, "ten") + phobos, "fear." The adjective form is triskaidekaphobic. One who fears the number 13 is a triskaidekaphobe or triskaidekaphobic.

There are many theories about the origin of triskaidekaphobia. In medieval Christian countries the number 13 came to be considered unlucky because there were 13 persons at the Last Supper of Christ. Fridays are also unlucky, because the Crucifixion was on a Friday. Hence a Friday falling on the thirteenth day would be regarded as especially unlucky.

Some famous triskaidekaphobes¹:
Napoleon
Herbert Hoover
Mark Twain
Richard Wagner
Franklin Roosevelt

1. Source: "It's just bad luck that the 13th is so often a Friday," Daily Telegraph, September 8, 1996