Friday the 13th

Why is Friday the 13th considered bad luck? Is it because 13 is considered an unlucky number (considered irregular, one more than the number of Greek gods, signs of the zodiac, numbers of numbers on a clock. Also the number of people who sat down at the Last Supper and one of them died), and Friday is considered an unlucky day (stock market crashes related to Black Friday, perhaps Jesus was crucified on a Friday?). I don't know. But so far nothing bad has happened to me. *fingers crossed*