Every now and then, the 13th day of the month happens to fall on a Friday. This can happen between one and three times a year. Friday the 13th is associated with a great deal of superstition in many cultures, and some people think that it is an unlucky or inauspicious day. So many people have fears about Friday the 13th that a word has been invented just to describe fear of Friday the 13th: paraskavedekatriaphobia. Say that five times fast, and once you've mastered it, you can move on to friggatriskaidekaphobia, a variant on “paraskavedekatriaphobia.”
Friday the 13th isn't unlucky everywhere. Generally speaking, superstitions about the day can be seen in French, German, and English-speaking cultures. Greeks, Spanish speakers, and Romanians, on the other hand, fear Tuesday the 13th, while the renegade Italians have their doubts about Friday the 17th. In some Asian cultures, four and seven are viewed as unlucky. In each case, the phobia is rooted in complex cultural traditions.
For people who fear this day, two fears are involved: fear of Fridays, and fear of the number 13. While it might seem strange to some people to fear a day of the week, fear of Fridays is actually very ancient. Numerous authors dating back to the medieval period noted that bad things happened on Fridays, and there were many superstitions about starting new endeavors on Fridays, traveling on Fridays, or really doing much of anything on a Friday. This fear dominates in Christian cultures, and is probably related to the belief that Christ was crucified on a Friday.
Fear of the number 13th is based in numerology. Some people feel that the number 13 is inauspicious. It is associated with Christ and the Apostles in some cultures, and with the idea that the number 12 is “whole,” while 13 is “out of balance.” A number of legends surround the number 13, perhaps most notably the tale that when 13 people sit down to dine, the first to get up will die. A variant on this myth has all of the unlucky diners perishing.
Curiously enough, despite the paired unease surrounding Fridays and the number 13, it wasn't until the 1800s that people started fearing Friday the 13th. Any time a month begins on a Sunday, people can look forward to a Friday the 13th. Some people genuinely do fear this day, and they may avoid activities on that day, and search for information which confirms their fear, such as news stories suggesting that day is especially busy for emergency rooms. Other people simply regard the legend as an old superstition, and they don't lend it much credence.