- In numerology, the number twelve is considered the number of completeness, as reflected in the twelve months of the year, twelve signs of the zodiac, twelve hours of the clock, twelve tribes of Israel, twelve Apostles of Jesus, twelve gods of Olympus, etc., whereas the number thirteen was considered irregular, transgressing this completeness. There is also a superstition, thought by some to derive from the Last Supper or a Norse myth, that having thirteen people seated at a table will result in the death of one of the diners.
- Friday has been considered an unlucky day at least since the 14th century's The Canterbury Tales, and many other professions have regarded Friday as an unlucky day to undertake journeys or begin new projects. Black Friday has been associated with stock market crashes and other disasters since the 1800s.It has also been suggested that Friday was the day that Jesus was crucified. 
Hammurabi's code omitted a 13th numbered item. The fear of 13, called triskaidekaphobia, has deep roots in numerology, 12 being the number of completion. It is thought that many millions of US citizens suffer from the fear of the "unlucky" Friday the 13th.
So for those of us who are reasonable people, happy Friday the 13th. It's arbitrary. It could be Friday. It could be called "wakenhuberday" for all we care. It could be the 13th of the month, or it could be the 44th day of the year. Just think how doomed we'd all be if 44 was also an unlucky number....