Happy Friday the 13th. Here's everything you never needed to know in the first place about today:
Fear of Friday the thirteenth (a fear recently dubbed paraskevidekatriaphobia), despite the fact that Friday turns out to be the most common weekday on which the 13th of a month can occur in the Gregorian calendar. The association of bad luck with Friday appeared in Chaucer's Canterbury Tales in the late 14th century ("and on a Friday fell all this misfortune"), but references to Friday as a day associated with ill luck in general first appear around the middle of the 17th century (Mikkelson and Mikkelson). In particular, it appeared in numerous publications as a particularly unlucky day to start a new venture (beginning a journey, giving birth, getting married, moving, starting a new job, etc.) beginning around 1800 (Mikkelson and Mikkelson).
While 13 is associated with the number of people at the Last Supper, the Crucifixion took place on a Friday, thus leading to an association of back luck with the combination of this number and day.
Paraskevidekatriaphobia is probably the most widespread superstition in the United States, possibly affecting tens of millions of Americans. Interestingly, a study by Scanlon et al. (1993) published in the prestigious British Medical Journal which analyzed the relation between health, behavior, and superstition surrounding Friday 13th in the United Kingdom by comparing the ratio of traffic volume to traffic accidents on Friday the 6th and Friday the 13th over a period of years found that, "The risk of hospital admission as a result of a transport accident may be increased by as much as 52%. Staying at home is recommended."
While there does not appear to be hard evidence to support the claim, it has been suggested that $800 to $900 million are lost each Friday the 13th (Roach 2004) as a result of people avoiding travel, wedding plans, moving, and so on.