A post on Yahoo's answer site had these little tidbits to share.

A Friday occurring on the 13th day of any month is considered to be a day of bad luck in English-speaking cultures around the globe. Similar superstitions exist in some other traditions. In Greece and Spain, for example, Tuesday the 13th takes the same role. The fear of Friday the 13th is called paraskavedekatriaphobia, a specialized form of triskaidekaphobia, a phobia (fear) of the number thirteen.

Strangely, there is evidence to suggest that Friday the 13th is actually unlucky. Psychologists have found that some people are especially likely to have accidents or fall ill on Friday the 13th [citation needed]. This has been attributed to such people feeling a heightened state of anxiety on that day. 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.

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