Today's vocabulary word is paraskavedekatriaphobia, the official term for the fear of Friday the 13th (the date, not the slasher-movie franchise — that would be crappifilmaphobia). It is an especially particular form of triskaidekaphobia, which is simply fear of the number 13. The origins of the latter phobia are tied to that of the first, which is to say nobody really knows where the superstition about 13 comes from, but it is pervasive in many cultures (though two-digit primes have always gotten a bum rap).
We can tell you that superstitions surrounding the confluence of Fridays and 13s didn't get documented until the 20th century, so there's little evidence that this particular phobia has deep historical roots. In fact, it isn't even that universal, as outside of Germany, Brazil, Portugal, and most English-speaking countries, Friday the 13th isn't considered unlucky — though in Greece, Tuesday the 13th has much the same connotation, as it was Tuesday, April 13, 1204 that Constantinople, the seat of the Ottoman Empire, fell to Christian forces in the Fourth Crusade. So, for 800 years, Greece has held a superstitious grudge against Tuesdays dated the 13th.
Thus, you can allay your fears about Friday the 13th with the knowledge that, until very recently and in only a very few countries, nobody gave it much thought. Fight the fad and treat today as a chance to humorously aggravate those poor souls who don't have your arrogantly dismissive knowledge of folklore and history.
Jay Garmon has a vast and terrifying knowledge of all things obscure, obtuse, and irrelevant. One day, he hopes to write science fiction, but for now he'll settle for something stranger — amusing and abusing IT pros. Read his full profile. You can also follow him on his personal blog.