Why is Friday the 13th considered unlucky?

I've no idea and I personally don't suffer from paraskavedekatriaphobia. But, if you do or are just curious about it, here's one theory from Cool Quiz.

The modern basis for the aura that surrounds Friday the 13th stems from Friday October the 13th, 1307. On this date, the Pope of the church in Rome in Conjunction with the King of France, carried out a secret death warrant Against "the Knights Templar". The Templars were terminated as heretics, never again to hold the power that they had held for so long. There [sic] Grand Master, Jacques DeMolay, was arrested and before he was killed, was tortured and crucified.

For any of you who do suffer from this phobia, help can be found here.