What I've read on the matter suggests that the Friday 13th superstition is directly related to the arrests of the Templars in France on Friday, October 13th, 1307. But there is some support for it having to do with the Norse gods numbering 12 at a feast in Valhalla when Loki, number 13 and uninvited, came in with the end result being the death of Balder, the god of joy.

In looking further I found this about.com article which seems to cover just about everything, http://urbanlegends.about.com/cs/his...the_13th_4.htm