1.13th guest at Last Supper
Judas, the apostle who betrayed Jesus, was the 13th guest at the Last Supper.
2. Crucifixion on Friday
In Christian tradition, Jesus was crucified on a Friday, which by that time was the day criminals were hanged, so Friday was considered a day of dread. Christian culture would in time combine the number 13 with Friday as an unlucky day.
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3. 13 is an incomplete number
In numerology, 12 is considered a complete number — 12 apostles, 12 major Greek gods, 12 hours in half a day, 12 inches in a foot, 12 jurors — whereas 13 is not.
4. Eve tempted Adam on a Friday
In biblical tradition, Eve tempted Adam with the forbidden fruit on a Friday.
5. Cain killed Abel on a Friday
Another Bible tradition holds that Cain murdered Abel on a Friday.