It’s Friday the 13th. For some, like Robert Pattinson, today is a great day. (May 13 is his birthday). But for many, Friday the 13th is associated with bad luck — and even horror.

Did you know that triskaidekaphobia is an extreme or irrational fear of the number 13? Here are some theories as to why people are especially suspicious about Friday the 13th. 

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13 has been considered an unlucky number for a long time

National Geographic reports that Donald Dossey, a folklore historian, believes the fear of the number 13 dates all the way back to Norse mythology. He recounted a story about the Norse gods getting together for a dinner. There were 12 guests. The Norse god Loki came as the 13th guest. Loki caused the god of darkness to shoot the god of gladness with a poisonous arrow. The god of gladness died, and the earth was covered in darkness. 

Some theorize that having 13 guests may be bad luck or may symbolize “courting death,” per the History channel

The number 13 is even unlucky in the Bible. In the New Testament, Jesus had 13 people at the last supper. One of those guests ended up being the one who sold him to the Pharisees. Jesus was killed soon after … on a Friday. In other biblical history, it is said that Eve gave Adam the apple on a Friday, and Cain was also killed on a Friday, per the History channel.

Pop culture has further perpetuated this myth with the creation of the “Friday the 13th” movies featuring Jason Vorhees. There are currently 12 of them, but there is some debate that there may be more on the way, although it isn’t currently clear, according to Looper. The newest unofficial “Friday the 13th” film, a fan film called “Friday the 13th Vengeance 2: Bloodlines,” has released today.