My research into the
Knights Templar indicates that the superstition began when the
King of
France had every known member of the Knights arrested on the suspicion of
heresy, on Friday, the 13th of October, 1307. This certainly was a rather
unlucky blow, to say the least; the Knights served valiantly in the
Crusades and are considered the creators of modern
banking, despite their personal indulgences.
The Templars faced interregation by torture, and most eventually confessed to sins ranging from usury to the worship of Baphomet, a Catholic rendering of the prophet Mohammed as a demon in the service of Satan.
This had a profound effect on the people of the time, who had seen the Templars go from a revered military order in the service of Christ to the most despised of all sinners, and the day of their downfall went down as an unlucky day indeed.