Friday the 13th
The stigma attached to Friday the 13th is widely believed to have biblical roots. The number 12 is seen in many cultures as a sort of “perfect” number and adding one more to that throws things off a bit.
According to the Bible, Judas was the 13th guest to arrive at the Last Supper and Friday was widely believed to be the day Jesus was crucified (but has since been brought into question.)
Similarly, in Norse mythology, Loki was the 13th guest to arrive at a dinner for the gods in Valhalla and wreaked havoc on the whole event. The Friday superstition also has origins in the U.S. where (in the 19th century) all executions took place on Friday.
There’s even a word for people who fear Friday the 13th: friggatriskaidekaphobia.