Why is it considered unlucky when the thirteenth day in a month falls on a Friday?

In North America and Europe, a significant portion of the population behaves very strangely on Friday the 13th. They won't fly in airplanes, host a party, apply for a job, get married or even start a new project. Some people won't even come into work. In the United States, roughly 8 percent of the population is afraid of Friday the 13th, a condition known as paraskevidekatriaphobia. "Friday the 13th" as we know it has its roots in many traditions and cultures.

The superstition surrounding Friday the 13th is actually a combination of two separate fears -- the fear of the number 13, called triskaidekaphobia, and the fear of Fridays. The most familiar source of both these phobias is Christian theology. Thirteen is significant to Christians because it is the number of people who were present at the Last Supper (Jesus and his 12 apostles). Judas, the apostle who betrayed Jesus, was the 13th member of the party to arrive.

Christians have traditionally been wary of Fridays because Jesus was crucified on a Friday. In addition to that, some theologians hold that Adam and Eve ate from the forbidden fruit on a Friday, and that the Great Flood began on a Friday. In the past, many Christians would never begin any new project or trip on a Friday, for fear that the endeavor would be doomed from the start.

Sailors were particularly superstitious in this regard, often refusing to ship out to sea on a Friday. According to legend, in the 18th century, the British Navy commissioned a ship called the H.M.S. Friday in order to quell the superstition. The navy selected the crew on a Friday, launched the ship on a Friday and even selected a man named James Friday as the ship's captain. Then, one Friday morning, the ship set off on its maiden voyage -- and disappeared forever.

Some historians trace the Christian distrust of Fridays to the church's overall opposition to pagan religions. Friday is named after Frigg, the Norse goddess of love and sex. This strong female figure, these historians claim, posed a threat to male-dominated Christianity. To fight her influence, the Christian church characterized her as a witch, vilifying the day named after her. This characterization may also have played a part in the fear of the number 13. It was said that Frigg would often join a coven of witches, normally a group of 12, bringing the total to 13. A similar Christian tradition holds that 13 is unholy because it signifies the gathering of 12 witches and the devil.

Some trace the infamy of the number 13 back to ancient Norse culture. In Norse mythology, the beloved hero Balder was killed at a banquet by the malevolent god Loki, who crashed the party of twelve, bringing the group to 13. This story, as well as the story of the Last Supper, led to one of the most entrenched connotations of the number 13: You should never sit down to a meal in a group of 13.

Another significant part of the Friday-the-13th legend is a particularly bad Friday the 13th that occurred in the middle ages. On a Friday the 13th in 1306, King Philip of France burned the revered Knights Templar at the stake, marking the occasion as a day of evil.

Some people come to fear Friday the 13th because of misfortune they've experienced on that day in the past. If you get in a car wreck on one Friday the 13th, or lose your wallet, that day is bound to stay with you. But if you think about it, bad things (from spilling your coffee to, well, much more serious problems) happen all the time, so if you're looking for bad luck on Friday the 13th, you'll probably find it.

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