The fear of Friday the 13th is known as paraskavedekatriaphobia. According to About.com's Urban Legends guide, an estimated 8 percent of Americans may suffer from the fear of Friday the 13th. Some people refuse to start new projects, go out to eat or even go to work on that date.
The fear of Friday the 13th is related to the fear of the number 13. That fear appears to be rooted in pre-Christian religious traditions, when the Norse god Loki invited himself to the Banquet of Valhalla, becoming the 13th guest. His mischief caused the death of Baldr, a favorite of the gods. Early Christian traditions also highlight 13 as an unlucky number. Christ's betrayer, Judas, may have been the 13th to join the table at the Last Supper.
Friday itself is also considered an unlucky day. In many pre-Christian sects, Friday was the Sabbath. Those who spent the day involved in their own matters did not properly honor the gods. In return, they could not expect the gods to bless their projects. Early Christians noted the day as the "Witches Sabbath," differentiating their own day of worship from that of the pagans. The Bible is sprinkled with references to catastrophic events that occurred on Fridays, from the Great Flood to the crucifixion of Christ.
Since both Friday and the number 13 are considered unlucky, it holds that the conjunction of the two is particularly unlucky. Some claim that this connection was solidified when the Knights Templar were arrested on Friday, Oct. 13, 1307. Yet the fear of Friday the 13th seems to be a much newer phenomenon, dating only to the beginning of the 20th century. About.com's Urban Legends guide posits that the connection between Friday and the number 13 may have been solidified by a popular 1907 book provocatively titled Friday the Thirteenth. Though it actually dealt with stock market corruption, the novel may have generated long-lasting interest and fear in the juxtaposition of day and date.Source:
American Psychiatric Association. (1994). Diagnostic and statistical manual of mental disorders (4th Ed.). Washington, DC: Author.