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The superstition was partly inspired by the Bible
According to the Bible, there were 13 guests at the Last Supper — Jesus and his 12 apostles, including Judas who eventually betrayed him. That sparked a superstition regarding 13 guests at a dinner table, and slowly that belief extended to the number itself being bad luck.
As for Friday, Jesus was crucified on that day, and so people began feeling uneasy about it.
