QUOTE(Victoria Silverwolf @ Apr 16 2007, 11:11 PM)

Although Friday and the number 13 have been considered unlucky for some time, there is no evidence that "Friday the Thirteenth" was considered as particularly unlucky until the 20th Century.
Even the quote provided in your link refers to it as an "ancient superstition"
13 has been considered unlucky because it is one more than 12.
Witches gathering in 12's with the 13th being the devil.
A gathering of 12 Norse gods interrupted by a 13th - Loki.
12, I guess, is a good number.
Fridays have long been considered unlucky. So, a Friday falling on the 13th would be considered very unlucky.
We'll get another one in July, so brace yourselves.