Fear of Friday the 13th is rooted in ancient, separate bad-luck associations with the number 13 and the day Friday.
The two unlucky entities combine to make one super unlucky day.
There is a Norse myth about 12 gods having a dinner party at Valhalla, their heaven. In walked the uninvited 13th guest, the mischievous 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 Earth got dark. The whole Earth mourned.
There is a Biblical reference to the unlucky number 13. Judas, the apostle who betrayed Jesus, was the 13th guest to the Last Supper.
A particularly bad Friday the 13th occurred in the middle ages. On a Friday the 13th in 1306, King Philip of France arrested the revered Knights Templar and began torturing them, marking the occasion as a day of evil.
In ancient Rome, witches reportedly gathered in groups of 12. The 13th was believed to be the devil.
Both Friday and the number 13 were once closely associated with capital punishment. In British tradition, Friday was the conventional day for public hangings, and there were supposedly 13 steps leading up to the noose.
It is traditionally believed that Eve tempted Adam with the apple on a Friday.
Tradition also has it that the Flood in the Bible, the confusion at the Tower of Babel, and the death of Jesus Christ all took place on Friday.
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.
In exceeding 12 by 1, 13's association with bad luck has to do with just being a little beyond completeness.
Jo said... If I remember right the Knights Templar were rounded up on Friday the 13th in 1307. I can't remember if it was in January or October though. According to history it was an organized effort launched in several countries on the same day so as not to warn the Templars ahead of time. That is also where the saying "Friday the 13th, unlucky for some" came from (the saying)
More than 80 percent of high-rises lack a 13th floor.
Many airports skip the 13th gate.
Airplanes have no 13th aisle.
Hospitals and hotels regularly have no room number 13.
Italians omit the number 13 from their national lottery.
On streets in Florence, Italy, the house between number 12 and 14 is addressed as 12 and a half.
Many cities do not have a 13th Street or a 13th Avenue.
Source: Corsinet.com - Trivia Sphere: Related Content
On Friday 13th October 1307, King Philip IV of France, who was financially in debt to the Templars, had Templar Grand Master Jacques de Molay and many other French Templars arrested for heresy and tortured until they 'confessed'. The publicity of these confessions caused Pope Clement to instruct all Christian monarchs in Europe to arrest all Templars and seize their assets.
After the torture was over, many Templars withdrew their confessions, but they were used anyway to have dozens burned at the stake in 1310.
The Grand Master Jacques de Molay was burned at the stake on 18th March 1314. Legend had it that he called out from the flames that both Pope Clement and King Philip would soon meet him before God. Pope Clement died only a month later and King Philip died in a hunting accident before the end of the year.