Today is Friday the 13th. How strong are superstitions regarding Friday the 13th? According to this Wikipedia entry:
The Stress Management Center and Phobia Institute in Asheville, North Carolina estimates that in the United States alone, $800 or $900 million is lost in business each Friday the 13th because some people will not travel or go to work.
Interestingly, the anxiety caused by Friday the 13th means that the day turns out to be unlucky for some.
Psychologists have found that some people are especially likely to have accidents or fall ill on Friday the 13th.
Those who have a fear of Friday the 13th are called "triskaidekaphobes". How should triskaidekaphobes combat their fear? According to a National Geographic article ("Friday the 13th Phobia Rooted in Ancient History"):
They need to realize that they have the ability to create much of their own good and bad luck," he said. "And they should concentrate on being lucky by, for example, looking on the bright side of events in their lives, remembering the good things that have happened, and, most of all, be[ing] prepared to take control of their future. - Richard Wiseman, a psychologist at the University of Hertfordshire in Hatfield, England
OK, as they say in The Hitchhiker's Guide to the Galaxy, "Don't Panic!" But what is the historical root of the Friday the 13th fear? One theory, posted on About.com, says:
Primitive man had only his 10 fingers and two feet to represent units, so he could not count higher than 12, according to this explanation. What lay beyond that — 13 — was an impenetrable mystery, hence an object of superstition.
Yet many other theories abound, according to the Wikipedia:
The origins of the Friday the 13th superstition has been linked to the fact there were 13 people at the last supper of Jesus, who was traditionally crucified on Good Friday, but it probably originated only in medieval times. It has also been linked to the fact that a lunisolar calendar must have 13 months in some years, while the solar Gregorian calendar and lunar Islamic calendar always have 12 months in a year. Another suggestion is that the belief originated in a Norse myth about twelve gods having a feast in Valhalla. The mischievous Loki gatecrashed the party as an uninvited 13th guest and arranged for Hod, the blind god of darkness, to shoot Baldur, the god of joy and gladness, with a mistletoe-tipped arrow. Baldur was killed and the Earth was plunged into darkness and mourning as a result.
Some also say that the arrest of Jaques de Molay, Grand Master of the Knights Templar, and 60 of his senior knights on Friday, October 13, 1307 by King Philip IV of France is the origin of this superstition. That day thousands of Templars were arrested and subsequently tortured. They then 'confessed' and were executed. From that day on, Friday the 13th was considered by followers of the Templars as an evil and unlucky day.