Triskadaphobia (from Greek tris meaning “3”, kai meaning “and”, deka meaning “10” and phobos meaning “fear” or “morbid fear”) is fear of the number 13 and avoidance to use it; it is a superstition and related to a specific fear of Friday the 13th, called paraskevidekatriaphobia (from Παρασκευή Paraskevi, Greek for Friday) or friggatriskaidekaphobia (after Frigg, the Norse goddess Friday is named after in English).
There is a myth that the earliest reference to thirteen being unlucky or evil is from the Babylonian Code of Hammurabi (circa 1780 BCE), where the thirteenth law is omitted. In fact, the original Code of Hammurabi has no numeration.
Other translations of the Code of Hammurabi, for example the translation by Robert Francis Harper, include the 13th article.
Some Christian traditions have it that at the Last Supper, Judas, the disciple who betrayed Jesus, was the 13th to sit at the table. However, the Bible itself says nothing about the order at which the Apostles sat. Also, the number 13 is not uniformly bad in the Judeo-Christian tradition. For example, the 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 was the 13th god in the Norse pantheon—more specifically, Loki was believed to have engineered the murder of Balder and was the 13th guest to arrive at the funeral. This is perhaps related to the superstition that if 13 people gather, one of them will die in the following year. 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), they cursed the infant by magically binding his lifespan to that of a mystic candle they presented to him.
Similarly, in the Grimm’s version of Sleeping Beauty, the wicked fairy is the thirteenth fairy.
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 number is identified with chaos and the reason Persians leave their houses to avoid bad luck on the thirteenth day of the Persian Calendar, a tradition called Sizdah Bedar.
In some regions 13 is considered a lucky number. For example, 13 is lucky in Italy except in some contexts, such as sitting at the dinner table. Colgate University was started by 13 men with $13 and 13 prayers, so 13 is considered a lucky number. Friday the 13th is the luckiest day at Colgate.