Friday the 13th

Friday the 13th is considered an unlucky day in Western superstition. It occurs when the 13th day of the month in the Gregorian calendar falls on a Friday, which happens at least once every year but can occur up to three times in the same year.
For a month to have a Friday the 13th, the first day of the month must be a Sunday.

One source mentioned for the unlucky reputation of the number 13 is a Norse myth about twelve gods having a dinner party in Valhalla. The trickster god Loki, who was not invited, arrived as the thirteenth guest, and arranged for Höðr, the god of darkness, to shoot Balder, the god of joy and gladness, with a mistletoe-tipped arrow. Balder died, triggering much suffering in the world, which caused the number 13 to be considered unlucky.