Why is Friday the 13th considered unlucky?

The number 13 has long been seen as awkward because it comes right after 12, a number linked with completeness: 12 months, 12 zodiac signs, 12 apostles. Stories about a doomed 13th guest at a meal, such as Judas at the Last Supper or the trickster god Loki at a Norse feast, strengthened the idea that 13 brings bad luck. Friday also had a gloomy reputation in Christian tradition as the day of the Crucifixion. The two ideas were not widely linked until the 1800s, and Thomas W. Lawson’s 1907 novel Friday, the Thirteenth helped turn the date into a pop-culture fear. Today many hotels and tower blocks still skip a 13th floor, and every calendar year has at least one Friday the 13th and never more than three.