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.
2015 had a Friday the 13th in February, March, and November,
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.
Unluckiness of "13"
One source mentioned for the unlucky nature of the number "13" is 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.
"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.
The superstition seems to relate to various things, like the story of Jesus' last supper and crucifixion in which there were 13 individuals present in the Upper Room on the 13th of Nisan Maundy Thursday, the night before his death on Good Friday.
