In a lot of American buildings, there is no 13th floor.  It's bad luck! At least once (but as many as three times) a year, Friday the 13th appears on the Gregorian calendar.  Traditionally this is supposed to be a day of bad luck, but just where that superstition comes from isn’t quite clear.  One popular belief is that it dates to Friday, October 13, 1307.  Legend has it that King Philip VI of France gave an order that a number of the Knights Templar were to be arrested on this date.  The arrests were supposed to occur simultaneously, to prevent them from resisting.  King Philip did indeed do this, but the association of that date with this act was not noticed until the early 20th century, so it’s unlikely there’s a connection.  Another theory is that the Last Supper had thirteen people in attendance, and according to Biblical accounts, it was one of the thirteen (Judas) who betrayed Christ, who was arrested and crucified by the Romans the next day, which was Good Friday.  T