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 Friday the 13th in February, March, and November, which will happen again in 2026; 2017 through 2020 had two Friday the 13ths, as did 2023; 2016, 2021 and 2022 had just one Friday the 13th, as will 2025, 2027, and 2028; 2024 will have two Friday the 13ths.

Friday is the day of the week between Thursday and Saturday. In countries that adopt the traditional "Sunday-first" convention, it is the sixth day of the week. In countries adopting the ISO 8601-defined "Monday-first" convention, it is the fifth day of the week.