Unless you’ve been living under a rock or in some remote Amazonian jungle village, you know that Friday the 13^th is considered unlucky. This irrational fear of the date is called paraskevidekatriaphobia, and it’s more widespread than you might realize. In fact, some experts claim it’s the most widely believed superstition in the U.S. Dr. Donald Dossey, a psychotherapist who specializes in phobias, states that some 21 million Americans are affected.

How did this come to be? How come Friday the 13^th is so scary? Well, of course, part of is the “13” itself. This has long been considered an unlucky number by several cultures. For example, the ancient Egyptians believe that humans went through twelve stages on earth, and the thirteenth one was the happy afterlife. While they didn’t see 13 as unlucky, it was often associated with death. This association spread to other cultures that ignored the Egyptians’ positive association with 13.

There are also all sorts of superstitions and folklore surrounding the number 13 because women generally experience 13 menses throughout a year. As a female, I can certainly see how this would be considered bad. But that’s not what the ancients were thinking about. When the solar calendar, with its 12 months, replaced the lunar calendar, with its 13 months, the number 13 was looked down upon.

Another mention of 13 being unlucky comes from Norse mythology and the story involving Loki. When the 11 great gods were invited by Odin to a feast at Valhalla, Loki was not invited because of his penchant for mischief. Loki learned about the banquet and showed up, anyway. He instigated a scuffle between Hod and Balder the Good, which ended in Balder’s death. This is most likely how the superstition about having 13 guests at a dinner table began.

There were 13 seated at the table at perhaps the most famous dinner in history, the Last Supper. Judas the betrayer was the 13^th guest, and we all know how that ended.

Also, traditionally speaking, there are 13 witches in a coven.

Okay, so all that sort of explains the negative aspects of the number 13. But what about Friday? Today, most people who work love Fridays. It’s the last day of the work week and the beginning of the weekend. Many places of employment even have “casual Fridays.” This auspicious day, however, was not always held in such high regard.

Let’s re-visit Norse mythology. Freya was the goddess of fertility, and her sacred animal was the cat. Our sixth day of the week, Friday, was named for her. Because of this, some ancient cultures believed that Friday was the best day for a wedding. With the rise of Christianity in Europe, pagan customs and legends were frowned upon, and cats were often seen as accomplices of evil. Freya, along with her day, fell from grace. Accordingly, Friday came to be known as “the witches’ Sabbath” in Scandinavian countries.

Part of the superstition about Friday is based on actual historical events. For one thing, that was the typical day set aside for executions in ancient Rome. According to the Bible, Christ was crucified on a Friday at the hands of the Romans. For years, executions in Britain were conducted on Fridays, too.

In 1066, on Friday, October 13, the final stage was set for the Battle of Hastings. That’s the day that William of Normandy demanded that Harold II of England hand over the throne to him. Harold declined, and the English troops fell to William’s army the next day. William the Conqueror took over Britain and changed the lives of the Anglo-Saxons forever. It’s understandable how they would view Friday the 13^th with dread.

Fast forward about 300 years to The Canterbury Tales. This collection of narrative verse was written by Geoffrey Chaucer in the fourteenth century and remains one of the most famous literary works ever written. Part of its appeal lies in the fact that readers get an understanding of what life was like during the period, including the customs, traditions, and beliefs. In “The Nun’s Priest’s Tale,” Chaucer says that Chauntecleer’s misfortunes took place on a Friday:

“ O destiny, you cannot be eschewed!

Alas, that Chauntecleer flew from the beams!

Alas, his wife recked nothing of his dreams!

And on a Friday fell all this mischance.”

The only significance of the reason the poet mentioned the specific day of the week must be because it held some special meaning for medieval audiences. This is typical of Chaucer, as he also mentions gapped teeth as being a sign of libido, an allusion lost on modern readers.

In the next few centuries, Friday was often seen as an unlucky day. By the 1600s, for example, Friday was viewed as a bad day for beginnings – starting out on a journey, getting married, beginning a new job or work project, being born, or moving to a new town.

In more modern times, stock market crashes have taken place on Fridays, hence the term “Black Friday.”

So, if we add all the superstition surrounding the number 13 to the fear of Friday, we have Friday the 13^th representing a “double threat.”

How serious is this old superstition taken? Many high rises don’t have a 13^th floor. Airports don’t have a Gate 13. Many towns skip from 12 to 14 when naming their streets. Lots of hotels avoid having a room #13. Airplanes lack a 13^th aisle. It used to be that in France, a 14^th dinner guest was always kept on standby in case 13 diners showed up to a dinner party. The national lottery of Italy does not use a number 13.

How often do we have Fridays that fall on the 13^th of the month? It changes from year to year. There can be from one to three Friday the thirteenths in a year. 2009 has three such days, in February, March, and November. The next year to have as many Friday the thirteenths is 2012.