Some claim the date has has a biblical significance, representing misfortune.
Jesus was crucified on a Friday and ever since the day has been associated with “bad omens”, says Michael Bailey, a history professor at Iowa State University who specializes in the origins of superstition. “Weddings in the Middle Ages, for instance, were not held on Fridays and it was not a day someone would start a journey”, Bailey said.
Thirteen guests are believed to have attended the Last Supper, according to Stuart Vyse, a psychology professor at Connecticut College. “Judas Iscariot, the disciple who betrayed Jesus, is commonly considered to have been the 13th guest”, Vyse said.
However, many historians claim the date refers to the day the Knights Templar were slaughtered.
After becoming a massively powerful religious order with both military and economic interests throughout Europe, Pope Clement V decided that the Knights Templar were a threat. He famously issued an order “Dieu n’est pas content, nous avons des ennemis de la foi dans le Royaume” (“God is not pleased. We have enemies of the faith in the kingdom.”). This was at the request of King Philip of France who was deeply in debt to the Knights Templar. On Friday October 13th, 1307, the Pope ordered the arrest of all Templar Knights including Templar Grand Master Jacques de Molay and the Hospitaller Grand Master Fulk de Villaret who were called to a meeting. The result was horrible. Hundreds of knights were captured, tortured, and killed. The massacre reached all across Europe where the once all powerful and Knights were forced to flee into exile, or be killed. Most Templar Knights were executed on sight, but some (and a small fleet of ships loaded with precious cargo) sailed off never to be heard from again.
The fear of the number 13 has been given a scientific name: “triskaidekaphobia“; the analogy to this the fear of Friday the 13th is called paraskevidekatriaphobia, from the Greek words Paraskeví (Παρασκευή, meaning “Friday”), and dekatreís (δεκατρείς, meaning “thirteen”)