Fear of Friday the 13th is called...?
A morbid, irrational fear of Friday the 13th. The fear of Friday the 13th is called friggatriskaidekaphobia (Frigga being the name of the Norse goddess for whom “Friday” is named and triskaidekaphobia meaning fear of the number thirteen.
According to folklore, historian Donald Dossey, the unlucky nature of the number "13" originated with a Norse myth about 12 gods having a dinner party in Valhalla. The trickster god Loki, who was not invited, arrived as the 13th guest, and arranged for Höðr to shoot Balder with a mistletoe-tipped arrow. Dossey: "Balder died, and the whole Earth got dark. The whole Earth mourned. It was a bad, unlucky day." This major event in Norse mythology caused the number 13 to be considered unlucky.