They date back to at least ancient Roman times, but Friday the 13th superstitions won't be getting much of a workout this year. Luckily for triskaidekaphobia sufferers, 2011—like 2010 before it—has only one Friday the 13th.

By contrast, 2009 boasted three Friday the 13ths—the maximum possible in a year, at least as long as we continue to mark time with the Gregorian calendar, which Pope Gregory XIII ordered the Catholic Church to adopt in 1582.

"You can't have any [years] with none, and you can't have any with four, because of our funny calendar," said Underwood Dudley, a professor emeritus of mathematics at DePauw University in Indiana, and author of Numerology: Or, What Pythagoras Wrought.

The calendar works just as its predecessor, the Julian calendar, did, with a leap year every four years. But the Gregorian calendar skips leap years on century years except those divisible by 400. For example, there was no leap year in 1900, but there was one in 2000. This trick keeps the calendar in tune with the seasons.

The result is an ordering of days and dates that repeats itself every 400 years, Dudley noted. As time marches through the order, some years appear with three Friday the 13ths. Other years have two or, like 2011, one.

Curious Calendar Encourages Friday the 13th Superstitions



"It's just that curious way our calendar is constructed, with 28 days in February and all those 30s and 31s," Dudley said.

When the 400-year order is laid out, another revelation occurs: The 13th falls on Friday more often than any other day of the week. "It's just a funny coincidence," Dudley said.

Richard Beveridge, a mathematics instructor at Clatsop Community College in Oregon, authored a 2003 paper in the journal Mathematical Connections on the mathematics of Friday the 13th.

He noted the 400-year cycle is further broken down into periods of either 28 or 40 years.

"At the end of every cycle you get a year with three Friday the 13ths the year before the last year in the cycle … and you also get one on the tenth year of all the cycles," he said.

2009, for example, was the tenth year of the cycle that started in 2000.

Friday the 13th Superstitions Linked to Triskaidekaphobia

Friday the 13th superstitions are rooted in ancient bad-luck associations with the number 13 and the day Friday, said Donald Dossey, a folklore historian and author of Holiday Folklore, Phobias and Fun.

The two unlucky entities ultimately combined to make one superunlucky day.

Dossey traces the fear of the number 13—aka, triskaidekaphobia—to a Norse myth about 12 gods having a dinner party at Valhalla, Norse mythology's heaven. In walked the uninvited 13th guest, the mischievous god Loki. Once there, Loki arranged for Hoder, the blind god of darkness, to shoot Balder the Beautiful, the god of joy and gladness, with a mistletoe-tipped arrow.

"Balder died, and the whole Earth got dark. The whole Earth mourned. It was a bad, unlucky day," Dossey said.

There is also a biblical reference to the unlucky number 13. Judas, the apostle said to have betrayed Jesus, was the 13th guest to the Last Supper. (See "Lost Gospel Revealed; Says Jesus Asked Judas to Betray Him.")

As for Friday, it's well known among Christians as the day Jesus was crucified. Also, some biblical scholars believe Eve tempted Adam with the forbidden fruit on Friday. Perhaps most significant is a belief that Abel was slain by his brother Cain on Friday the 13th.

Meanwhile, in ancient Rome, witches reportedly gathered in groups of 12. The 13th was believed to be the devil.

In modern times, many triskaidekaphobes point to the ill-fated mission to the moon, Apollo 13.

Thomas Fernsler, an associate policy scientist in the Mathematics and Science Education Resource Center at the University of Delaware in Newark, said the number 13 suffers because of its position after 12.

According to Fernsler, numerologists consider 12 a "complete" number. There are 12 months in a year, 12 signs of the zodiac, 12 gods of Olympus, 12 labors of Hercules, 12 tribes of Israel, and 12 apostles of Jesus, he pointed out.

Thirteen's association with bad luck, he said, "has to do with just being a little beyond completeness. The number becomes restless or squirmy"—not unlike some folks with triskaidekaphobia today.

Paralyzed by Friday the 13th Superstitions



Some people are so paralyzed by Friday the 13th superstitions that they refuse to fly, buy a house, or act on a stock tip, for example.

"It's been estimated that [U.S] $800 or $900 million is lost in business on this day, because people will not fly or do business they would normally do," said Dossey, the historian, who is also the founder of the Stress Management Center and Phobia Institute in Asheville, North Carolina.

Among other services, Dossey's organization counsels clients on how to conquer Friday the 13th superstitions, which fuel a phobia that he estimates afflicts 17 to 21 million people in the United States.

Symptoms range from mild anxiety to full-blown panic attacks. The latter may cause people to reshuffle schedules or miss an entire day's work.

When it comes to bad luck of any kind, Richard Wiseman—a psychologist at the University of Hertfordshire in Hatfield, England—has found that people who consider themselves unfortunate are more likely to believe in superstitions associated with bad luck.

Supersitious people's "beliefs and behavior are likely to be part of a much bigger worldview," he said. "They will believe [both] that luck is a magical force and that it can ruin their lives."

Triskaidekaphobia's Architectural Effects

Triskaidekaphobia can be seen even in how societies are built. More than 80 percent of high-rise buildings lack a 13th floor. Many airports skip the 13th gate. Hospitals and hotels regularly have no room number 13.

On streets in Florence, Italy, the house between number 12 and 14 is addressed as 12 1/2. In France socialites known as the quatorziens ("fourteeners") once made themselves available as 14th guests to keep a dinner party from an unlucky fate.

DePauw University's Dudley said nobody really knows why Friday the 13th has spawned so many superstitions.