Nervous about Friday the 13th?

Today is the second of three Friday the 13ths in 2012 — all, ominously, 13 weeks apart.

Everyone knows that Friday the 13th is unlucky, of course. We learned it from our families, from grade-school teachers, from a series of objectively awful horror movies.

Most of us don't buy into the superstition, but some take it very seriously indeed. There are people who refuse to go to work, drive a car or even get out of bed on Friday the 13th. But why is Friday the 13th supposedly so unlucky?

Turns out there's no easy answer. The origins of the superstition are shrouded in folklore. According to Wikipedia, there's no written evidence of a specific "Friday the 13th" superstition before the 19th century — but Friday has been considered an unlucky day, and 13 an unlucky number, for much longer. Friday has been considered unlucky at least as far back as the 14th Century (it's mentioned in The Canterbury Tales).

As for the number 13, some believe it's considered unlucky because there were 13 attendees at the Last Supper, one of whom betrayed Jesus (and Jesus was, of course, crucified on a Friday). Others point to the Norse pantheon, in which the 13th god was Loki, god of mischief and discord.

Silly or not, triskaidekaphobia influences society in many ways. Here, courtesy of the Huffington Post, are some ways in which the number 13 crops up — or is consciously avoided — in everyday life.

• Many hotels and and hospitals regularly have no Room 13
• Many cities don't have a 13th Street or 13th Avenue
• Many airports don't have a 13th gate
• Airplanes have no Aisle 13
• More than 80 percent of high-rises have no 13th floor
• A hangman's knot traditionally had 13 loops and a gallows 13 steps
• The 13th card in a tarot deck is Death

Are you wary of Friday the 13th? Why or why not? Tell us in comments.