Triskaidekaphobia (from Greek tris=three, kai=and, deka=ten) is an irrational fear of the number 13; it is a superstition and related to a specific fear of Friday the 13th, called paraskevidekatriaphobia or friggatriskaidekaphobia.
Some Christian traditions have it that at the Last Supper, Judas, the disciple who betrayed Jesus, was the 13th to sit at the table , and that for this reason 13 is considered to carry a curse of sorts. However, the number 13 is not uniformly bad in the Judeo-Christian tradition. For example, the 13 attributes of God (also called the thirteen attributes of mercy) are enumerated in the Torah (Exodus 34: 6-7). Some modern Christian churches also use 13 attributes of God in sermons.
Triskaidekaphobia may have also affected the Vikings—it is believed that Loki in the Norse pantheon was the 13th god. More specifically, Loki was believed to have engineered the murder of Baldr, and was the 13th guest to arrive at the funeral. This is perhaps related to the superstition that if thirteen people gather, one of them will die in the following year. This was later Christianized in some traditions into saying that Satan was the 13th angel. Another Norse tradition involves the myth of Norna-Gest: when the uninvited norns showed up at his birthday celebration—thus increasing the number of guests from ten to thirteen—the norns cursed the infant by magically binding his lifespan to that of a mystic candle they presented to him.
The Mesopotamian Code of Hammurabi (ca. 1760 BC) omits 13 in its numbered list. This seems to indicate a superstition existed long before the Christian era. Ancient Persians believed the twelve constellations in the Zodiac controlled the months of the year, and each ruled the earth for a thousand years at the end of which the sky and earth collapsed in chaos. Therefore, the thirteenth is identified with chaos and the reason Persian leave their houses to avoid bad luck on the thirteenth day of the Persian Calendar (a tradition called Sizdah Bedar).
In 1881, an influential group of New Yorkers led by U.S. Civil War veteran Captain William Fowler came together to put an end to this and other superstitions. They formed a dinner club, which they called the Thirteen Club. At the first meeting, on Friday 13 January 1881 at 8:13 p.m., 13 people sat down to dine in room 13 of the venue. The guests walked under a ladder to enter the room and were seated among piles of spilled salt. All of the guests survived. Thirteen Clubs sprang up all over North America for the next 40 years. Their activities were regularly reported in leading newspapers, and their numbers included five future U.S. presidents, from Chester A. Arthur to Theodore Roosevelt. Thirteen Clubs had various imitators, but they all gradually faded from interest as people became less superstitious.