There is a myth that when Christianity came and the Norse Gods were cast out, Freya was declared a witch. She was believed to meet with twelve other witches, making a total or 13, on the day named after her, Friday.

This meant the number 13 coinciding with the day Friday is considered to be something to be superstitious about.

Evt:

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.