This Friday the 13th, watch out for deep sea serpents and deathwatch beetles, and even something as innocent as a rainbow. The origins of Friday the 13th are murky, with possible connections to Judas being the 13th guest at the Last Supper, or the Knights Templar being condemned on a Friday the 13th. Just like the holiday, many superstitions have mysterious origins. We often hang on to old ideas without knowing how they came about.