Friday the 13th. The unlucky day. But why do people think Friday 13th is unlucky? No one really knows. Why would anyone even think a particular day is unlucky? Stuart Vyse, the author of Believing in Magic: The Psychology of Superstition, says that superstitions stem out of a desire for control over our lives: so if we think there is a particular day on which the universe is out to get us, we're going to be more careful. And if something bad happens to you on Friday 13th, you can blame the malevolent forces of bad luck. The concept of an unlucky day might have begun with the Ancient Romans, who divided their entire calendar into comitalia (lucky days) fasti (regular days) and nefasti (unlucky days). On nefasti a Roman tried to limit their activities. A Roman general would never consider fighting a battle on a nefastus. Nor would a priest conduct religious ceremonies. A Roman merchant who needed to sign a contract would wait for a faustus, or preferably a comitalis. Avoiding nefasti gave the Romans the illusion they were doing something to prevent misfortune. But why Friday? Why the 13th? No one is sure. Some say it's because Friday was the day Jesus was crucified and that Judas Iscariot was his 13th disciple. Others maintain that the superstition after the 1907 publication of Thomas W. Lawson's novel Friday, the Thirteenth about an unscrupulous broker's attempt to crash the stockmarket on that day. I haven't read it (and probably never will) but check out the cover (right). Isn't it menacing? Just looking at it makes me anxious about the rest of today.