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This sparked my interest, so I had to look it up. For anyone curious, seems that Jesus having 12 disciples seems to be one possible origin.
Seems weird since one of the 12 betrayed him, and maybe a thirteenth disciple would have exposed the plot and Jesus would have avoided crucifixion, thus sparing the world centuries of oppression and war.
The thirteenth was Mary Magdalene but we can't have a woman disciple.
My fundamentalist dad told me it was because the first passover (when God killed the first born son of every Egyptian household that didn't have lamb's blood) landed on Friday the 13th. Take that with a grain of salt.