The fear of Friday the 13th is called friggatriskaidekaphobia. Go ahead and say that fast.
Several theories have been proposed about the origin of the Friday the 13th superstition. One theory states that it is a modern amalgamation of two older superstitions: that thirteen is an unlucky number and that Friday is an unlucky day.
According to the esteemed source Wikipedia:
Friday has been considered an unlucky day at least since the 14th century's The Canterbury Tales, and many other professions have regarded Friday as an unlucky day to undertake journeys or begin new projects. Black Friday has been associated with stock market crashes and other disasters since the 1800s. It has also been suggested that Friday has been considered an unlucky day because, according to Christian scripture and tradition, Jesus was crucified on a Friday.