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 example, 2015 had a #Fridaythe13th in February, March, and November, which will happen again in 2026; 2017 through 2020 had two Friday the 13ths; 2016, 2021 and 2022 had just one Friday the 13th, as will 2025; 2023 and 2024 have two Friday the 13ths. A month has a Friday the 13th if and only if it begins on a Sunday. 
One source mentioned for the unlucky nature of the number "13" is a #Norsemyth 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