Exactly 716 years ago on Friday, 13th of October, 1307, the arrests of the Templars was carried out under the orders of Philip IV. This set in motion the ultimate destruction of the Templars. And perhaps gave rise to the superstition that any Friday 13th is considered unlucky.
Dienekes, remarked with a laugh, 'Good. Then we'll have our battle in the shade.' - Herodotus, The Histories, 7.226
Apparently, in the Bible bad things often happen on Fridays. (I wouldn't know.) But combine that with the 13th man at a certain event, and you have a likely source for the superstition. Never knew about the Templar angle until now though.