Horror films whose titles seem to bear no relationship to the story aside, Friday the 13th is a highly superstitious and fearful date for many. But where does this come from?
One source explains the general triskaidekaphobia as stemming from the 13th guest at the last supper being Judas, the betrayer.
As for the friday, that comes into play in ways less clear. Some researchers note that no written record of Friday the 13th as unlucky exists prior to the 19th century, specifically in Henry Sutherland Edwards' 1869 biography of Gioachino Rossini. But in some traditions, Friday has always been unlucky (check out the Canterbury Tales) and there are numerological reasons to find 13 unlucky as well; so, the two colliding represent a worst case scenario.
And while there is no historical support for Dan Brown's assertion in the popular novel The DaVinci Code that it has any Templar associations, the unsubstantiated belief is gaining credence among the ignorant.