I don't mean to discredit you, but there are actually other reasons that Friday the 13th is considered unlucky and many of them are the same reasons the number 13 is considered unlucky. The fact that bad things happen around the number 13 is a concept that is quite old. We spent an entire semester on the number 13 in my Arithmancy classes when I was a student. The origins are Norse and the addition of Friday became a superstition after the death of Jesus on a Friday though it was the 14th.