Triskaidekaphobia, the fear of 13 (of anything), is sometimes rationalized as "the number present at the Last Supper," the twelve apostles and Christ. Because that repast is assumed to have taken place on Friday, a Friday numbered 13 is doubly cursed, ominous of death, disaster, and doom of all kinds. And for paraskevidekatriaphobics, the fear of Friday the 13th locks them indoors, trembling under the bedclothes, during a date that occurs one to three times a year.
Even more intriguing, many paraskevidekatriaphobics are not Christian.
The rise of the compound Three-Ten for Thirteen is so very general all over the world, that it seems clear that to the primitive mind of early Man it had no real meaning—he stopped at Twelve. So persistent are these old instincts that, even today, we stop at "Twelve Times Twelve" in our school multiplication tables,... [due to an] inherited instinct that it was, and therefore still must be, the utmost limit of mathematical thought... Thirteen, therefore was not used as number, but as a vague word meaning anything beyond Twelve. To the untutored savage... anything unknown conveyed an immediate sense of danger. Thirteen was not really an unlucky number, but a fateful one—a number full of vague and unimaginable possibilities and therefore a number to be avoided by any peace-loving man.
On the other hand, ABC News online (in their Health section today) attributes the fear to Norse mythology, and notes that it costs 800 to 900 million dollars in lost business as people refuse to travel, shop or go to work.
Still, some think 13 owes its bad reputation originally to Loki, the Norse god of evil, who started a riot when he crashed a banquet at Valhalla attended by 12 gods. Once there, Loki (who then became the 13th god at the banquet) arranged for Hoder, the blind god of darkness, to shoot Balder the Beautiful, the god of joy and gladness, with a mistletoe-tipped arrow.
Lubo Motl gives us a rational (if low-odds) reason to fear one particular Friday of dire number. On Friday, April 13th, 2029, there was expected to be a 2.7 percent average chance that "an asteroid named 2004 MN4 of around 390 meters" would impact Earth at around 9:07 PM GMT. Fortunately for all of us, phobic or non, the probability was downgraded on December 27th, 2004 to 10^-5, a very tiny number, although still not zero.