One of the most commonly held superstitions in our presumably civilized, educated society is that Friday the 13th is an unlucky day. In this equation, both Friday and the number thirteen are held to be unlucky, so their sum can only equal double trouble.

Friggatriskaidekaphobes or paraskevidekatriaphobics are those afflicted with a morbid, irrational, overwhelming fear of Friday the 13th. Where does this unnatural trepidation of Friday the 13th originate? As the story goes, in order to understand thirteen, one has to understand the history of the number twelve. The number twelve has traditionally represented completeness. There are twelve months of the year, twelve gods of Olympus, twelve signs of the zodiac and twelve apostles of Jesus. Thirteen exists just one digit beyond twelve, and is symbolic of the first departure from completeness or the initial step towards evil.

The superstitious coupling of Friday the 13th with calamity is very old in western culture. The sixth day of the week and the number 13 both have foreboding reputations dating from ancient folklore; their inevitable conjunction from one to three times a year portends more misfortune than some credulous minds can bear. Folklorists say it's probably the most widespread superstition in America (and no doubt other parts of the world, as well). Some people won't go to work on Friday the 13th. Some won't eat in restaurants. Many wouldn't think of setting a wedding on the date.

The modern basis for the aura that surrounds Friday the 13th stems from Friday October the 13th, 1307. On this date, the Pope of the church in Rome in conjunction with the King of France, carried out a secret death warrant against "the Knights Templar". The Templars were terminated as heretics, never again to hold the power that they had held for so long. Their Grand Master, Jacques DeMolay, was arrested and before he was killed, was tortured and crucified.

Superstitions swirling around Friday as being lucky or unlucky have existed since ancient times, beginning with the northern nations. Ancient Romans dedicated the sixth day of the week to their beautiful but vain goddess Venus, so when the Norsemen adopted the Roman method of naming days, they naturally adopted Venus as their name for the sixth day of the week. Their closest translation for Venus, Frigg, or Freya, eventually evolved into Friday, a day they considered to be the luckiest day of the week. Friday was considered very lucky because of its associations with love and fertility. All that changed when Christianity came along. The goddess of the sixth day - most likely Freya in this context, given that the cat was her sacred animal - was recast in folklore as a witch, and her day became associated with evil doings.

Many legends developed in that vein, but one is of particular interest. As the story goes, the witches of the north used to observe their sabbath by gathering in a cemetery in the dark of the moon. On one such occasion the witch-goddess, Freya herself, came down from her sanctuary in the mountaintops and appeared before the group of witches - who numbered 12 at the time - and gave them one of her cats, making it ever afterward a coven of 13.

From a religious standpoint, Muslims tout Friday as the day Allah created Adam, legend has it that Adam and Eve ate the forbidden fruit, the apple, on a Friday, and later died on a Friday, and Christians consider Friday as the day on which Christ was crucified by the Romans.

The Scandinavian belief that the number 13 signified bad luck sprang from their mythological 12 demigods, who were joined by a 13th demigod, Loki, an evil cruel one, who brought upon humans great misfortune. The number 13, in the Christian faith, is the number of parties at the Last Supper, with the 13th guest at the table being the traitor, Judas. When Christians combine this day and number, the combination can only hold special significance.

It might be easy to laugh at such superstition, but this same kind of superstitious thinking operates to support beliefs that can have serious consequences. It is estimated that the 13th of the month costs America a billion dollars a year through train and plane reservation cancellations, absenteeism, and reduced commerce.

The Committee for the Scientific Investigation of Claims of the Paranormal routinely celebrates Friday the 13th with a party at which mirrors are broken, umbrellas are opened indoors, and other superstitious taboos are brashly challenged. On the other hand, we might be careful about tempting fate with such open defiance of practices steeped in superstition. One hundred years ago, the British government sought to debunk the widespread superstition among seamen that sailing on Friday was unlucky. A special ship was commissioned, to be named "H.M.S. Friday." They laid her keel on a Friday, launched her on a Friday, selected the crew on a Friday and put her in command of Captain Jim Friday. Finally, the H.M.S. Friday embarked on her maiden voyage - on a Friday - and was never seen or heard from again.