Watch out! Today is Friday the 13th, known by many as the unluckiest day of the year.

While many will laugh off the superstitious day, others will remain in bed paralyzed by fear and avoid daily tasks, conducting business or traveling. In the U.S., an estimated 17 to 21 million people suffer from a fear of Friday the 13th, according to a study by the North Carolina Stress Management Center and Phobia Institute.

The phobia, known as friggatriskaidekaphobia, is not uncommon. The word comes from Frigga, the name of the Norse goddess for whom Friday is named, and triskaidekaphobia, or fear of the number thirteen. It is also sometimes called paraskevidekatriaphobia, from the Greek Paraskevi for Friday, Dekatreis for thirteen and phobia for fear.

There will be three incidences of the superstitious day this year, Jan. 13, Apr. 13 and July 13. In the Gregorian calendar, Friday the 13th always occurs at least once a year and can appear up to three times in any one year.

History and Origins of Friday the 13th

The origin of fears surrounding Friday the 13th is unclear. There is reportedly no written evidence of Friday the 13th superstition before the 19th century, but superstitions surrounding the number 13 date back to at least 1700 BC.

In the ancient Babylon's Code of Hammurabi, dating to about 1772 BC, the number 13 is omitted in the list of laws.

There has also been a longstanding myth that if 13 people dine together, one will die within a year. The myth comes from both the Last Supper, when Jesus dined with the 12 Apostles prior to his death, and a popular Norse myth, in which 11 close friends of the god Odin dine together only to have the 12-person party crashed by a 13th person, Loki, the god of evil and turmoil.

In fact, the number 13 has been considered cursed across the world for thousands of years. The number 12 is historically considered the number of completeness, while its older cousin, 13, has been seen as an outlier. There are 12 months of the year, 12 gods of Olympus, 12 hours of the clock, 12 tribes of Israel, 12 Apostles of Jesus, 12 Descendants of Muhammad Imams, among many incidences of the pattern historically.

In 1881, an organization was started called The Thirteen Club in an attempt to improve the number's reputation. The 13 members walked under ladders and spilled salt at the first meeting in an attempt to dissuade any negative associations with the number.

Despite these efforts, the number 13 continues to have an unlucky association today. Thirteen is so disliked that many cities do not have a 13th Street or a 13th Avenue, many high-rise buildings avoid having a 13th floor, some hospitals avoid labeling rooms with the number 13 and many airports will not have a gate 13.

Friday has also long been considered an unlucky day. One theory hypothesizes that Friday has been considered unlucky because Jesus was crucified on a Friday according to Christian Scripture and tradition. Another states that the superstition regarding Friday comes from Chaucer's The Canterbury Tales, published in the 14th century, where Friday is considered a day of misfortune and ill luck. In numerous publications in the 17th century, Friday the 13th was outlined as an unlucky day to take a trip, to begin a new project or to have a major life change (such as a birth, a marriage, among other events).

The first recorded reference in English of Friday the 13th is in Henry Sutherland Edwards' 1869 biography of Gioachino Rissini, where Edwards writes: Rossini was surrounded to the last by admiring and affectionate friends; Why Friday the 13th Is Unlucky.

Nathaniel Lachenmeyer, author of Thirteen: the story of the world's most popular superstition, however, suggests in his book that because references to Friday the 13th were nonexistent before 1907, the popularity of the superstition must come from the publication of Thomas W. Lawson's popular novel, Friday, the Thirteenth. In the novel, a stock broker takes advantage of the superstition to create a Wall Street panic on Friday the 13th.

Wall Street has fostered a fear of Friday the 13th for decades. In Oct. 13, 1989, Wall Street saw, what was at the time, the second largest drop of the Dow Jones Industrial Average in history. The day was nicknamed the Friday-the-13th mini-crash.

Friday the 13th was also discussed in the popular 2003 novel, The Da Vinci Code. In the book, a connection is drawn between the slaughtering of the Knights Templar by the Church and Friday the 13th. Historically, the arrest of Jacques de Molay, the Grand Master of the Knights Templar, did occur on Friday, Oct. 13, 1307, however the association between Friday the 13th and bad luck is thought to be a modern interpretation of the event.

Superstition or Reality?

Many studies have been conducted to investigate the risk of accidents on Friday the 13th.

According to research completed at the Dutch Centre for Insurance Statistics (CVS) in 2008, there were fewer accidents and reports of theft or fire on Friday the 13th than on other Fridays.

I find it hard to believe that it is because people are preventatively more careful or just stay home, but statistically speaking, driving is a little bit safer on Friday 13th, CVS statistician Alex Hoen told the Verzekerd insurance magazine.

Between 2006 and 2008, CVS found that there were an average 7,800 traffic deaths on each Friday, but the average fell on Friday the 13th when there were only 7,500 deaths. Similar statistics were found in a comparison of normal Fridays and Friday the 13th for fires and robberies.

According to a 1993 study in the British Medical Journal, however, there is a significant level of traffic-related incidences on Friday the 13th as opposed to a random day, such as Friday the 6th, in the UK.

Friday 13th is unlucky for some. The risk of hospital admission as a result of a transport accident may be increased by as much as 52%. Staying at home is recommended, the report concluded.

According to CNBC, Friday the 13th is a calm day for the stock market, average gains proving to be only 0.2 percent or less. However, in three out of five of the most recent Friday the 13ths, CNBC reports that major averages ended down. The North Carolina Stress Management Center and Phobia Institute also reports that between 800 and 900 million dollars are lost in the U.S. each year due to shoppers remaining at home or deciding not to travel.

Popular Myths and Superstitions

Whether there is any merit to the superstitions surrounding Friday the 13th will remain uncertain, but that will not stop millions of people across the world from worrying about the unlucky day.

There are a number of popular myths and superstitions surrounding the day, most famously: