So how did Friday the 13th become such an unlucky day?

Dossey, also a folklore historian and author of Holiday Folklore, Phobias and Fun, said fear of Friday the 13th is rooted in ancient, separate bad-luck associations with the number 13 and the day Friday. The two unlucky entities ultimately combined to make one super unlucky day.

Dossey traces the fear of 13 to a Norse myth about 12 gods having a dinner party at Valhalla, their heaven. In walked the uninvited 13th guest, the mischievous Loki. Once there, Loki arranged for Hoder, the blind god of darkness, to shoot Balder the Beautiful, the god of joy and gladness, with a mistletoe-tipped arrow.

"Balder died and the whole Earth got dark. The whole Earth mourned. It was a bad, unlucky day," said Dossey. From that moment on, the number 13 has been considered ominous and foreboding.