We've all heard the tale that when the 13th day of the month falls on a Friday, bad things will happen. Friday the 13th is a harbinger of bad luck, right up there with black cats crossing your path or walking under ladders. There's even a horror movie by the same name! Fear of the number 13 has significance that dates back to ancient times. Here's one short theory of its origin from About.com:
It has been proposed, for example, that fears surrounding the number 13 are as ancient as the act of counting. Primitive man had only his 10 fingers and two feet to represent units, this explanation goes, so he could count no higher than 12. What lay beyond that — 13 — was an impenetrable mystery to our prehistoric forebears, hence an object of superstition.
There is even a name for having an irrational fear of Friday the 13th: paraskevidekatriaphobia. Try saying that three times fast! If you are afraid of the number 13 itself, you suffer from triskaidekaphobia.
For more information about the origins of superstitions related to the number 13, and other urban legends and phobias, check out these sources:
Why Friday the 13th is Unlucky. About.com.
Friday the 13th. Snopes.com.
How Friday the 13th Works. HowStuffWorks.com.
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(Image courtesy of the Encyclopedia Britannica database. "Munch, Edvard: The Scream." Online Photograph. Encyclopædia Britannica Online. 23 Jan. 2009 http://search.eb.com/eb/art-75415)