I did some digging into the origins of the Friday the 13th myth, and guess what... It's Christian misogyny. No... really.
Short version: 13 goes back to ancient timekeeping -- 13 x 28 lunar cycles... which, incidentally, is also women's menstrual cycle, which... as we all know, is pretty much evil if you're a Christian. Friday is Freya (Or... ironically, Frigg), a fertility goddess that was turned into a witch.
Long version: I've got a full length article on the subject at Atlanta Examiner.