The origins of Friggatriskaidekaphobia (the fear of Friday the 13th) are a little muddled, but it's often associated with two ideas: that thirteen is an unlucky number, and that Friday is an unlucky day.
In numerology, the number twelve is favored for its association with completeness: twelves months in a year, twelve hours on a clock, twelve Apostles, twelve tribes of Israel, twelve gods of Olympus, etc. Thirteen, then, is the perversion of this perfect completeness; twelve's a party, thirteen a crowd. Some believe that seating thirteen people at a table will result in the death of one, a superstition inspired by both The Last Supper and an old Norse myth.
But why Friday? Bad end-of-week vibes can be traced back to as early as the 14th century, in Geoffrey Chaucer's The Canterbury Tales. Stock market crashes and other disasters, since the 1800s, have been associated with Black Friday, and many believe it is inauspicious to begin projects, embark on journeys, or release products on––you guessed it!––Friday.
Friggatriskaidekaphobia affects an estimated 17-21 million people in the US, of which many are to scared to travel on planes, go to work, or even get out of bed. Either that, or it's just as good an excuse as any to play hooky.