According to biblical tradition, 13 guests attended the Last Supper, held on Maundy Thursday, including Jesus and his 12 apostles (one of whom, Judas, betrayed him). The next day, of course, was Good Friday, the day of Jesus’ crucifixion.
The seating arrangement at the Last Supper is believed to have given rise to a longstanding Christian superstition that having 13 guests at a table was a bad omen—specifically, that it was courting death.
Though Friday’s negative associations are weaker, some have suggested they also have roots in Christian tradition: Just as Jesus was crucified on a Friday, Friday was also said to be the day Eve gave Adam the fateful apple from the Tree of Knowledge, as well as the day Cain killed his brother, Abel.
On Friday, October 13, 1307, officers of King Philip IV of France arrested hundreds of the Knights Templar, a powerful religious and military order formed in the 12th century for the defense of the Holy Land.
Imprisoned on charges of various illegal behaviors (but really because the king wanted access to their financial resources), many Templars were later executed. Some cite the link with the Templars as the origin of the Friday the 13th superstition, but like many legends involving the Templars and their history, the truth remains murky.