There are a few explanations of just why 13 has developed the reputation for being unlucky:
- In Christianity, the Last Supper had thirteen attendants, during which Jesus Christ revealed that one of the guests would betray him. Legend tells that afterward one of the Apostles, Judas Iscariot, did so and turned Jesus over to the Romans. This led to the crucifixion of Jesus. It is also claimed that Judas sat at the thirteenth place at the Last Supper's table. As a result of this betrayal, the number 13 has maintained a bad reputation and has a superstitious characteristic in Christian culture.
- Bearing some similarity to the Christian tradition, in Norse lore, a banquet of twelve deities was interrupted by the evil god Loki, making thirteen the number of gods present when the nigh-immortal god Balder was killed by an arrow made from mistletoe (his only weakness). His death marked the beginning of Ragnarok, the end of everything.
- Thirteen is the number of circles used in Kaballah's Metatron's Cube. Although this glyph was typically used as a protective ward against demons and evil, its use in alchemy and magic may have led to a darker association with the form, much as a pentagram or pentacle has.
- In Meso-American divination, a trecena is a 13-day period used in pre-Columbian Meso-American calendars to mark the occurrence of important cycles of fortune/misfortune.
- The ancient Persians, assigning the twelve constellations of the Zodiac to the months of the year, believed that each sign ruled over Earth for a millennium. After the passing of this thirteenth era, chaos would ensue. The Persian tradition of Sizdah Bedar refers to this belief, in which people leave their houses to avoid bad luck on the thirteenth day of the Persian Calendar.
- The foiled Gunpowder Plot that threatened 17th Century England's government consisted of thirteen original conspirators.
- Apollo 13 was the only unsuccessful mission by the United States of America's NASA program to land a mission on the moon, although fortunately all of the craft's crew returned to Earth safely.
- Traditionally thirteen is reported as the number of steps leading up to a gallows.
- In numerology, twelve is considered the number of completeness and thus thirteen was seen as disturbing that balance,
- Witches were claimed to gather in groups (a coven) of twelve and during such ceremonies, the devil would appear as the thirteenth attendee.
- On Friday, 13 October 1307, Philip IV of France ordered the arrest of the (Christian) Knights Templar and seizure of all of their assets based on accusations of various forms of heresy. Some were later tortured and then burned at the stake. This is commonly believed to serve as the origin for the observance of Friday the 13th as an unlucky day, however, no record of the date's importance as a superstitious holiday is believed to exist before the 19th century.
- Friday the 13th is also theorized to originate from the claim that Jesus Christ was crucified on a Friday.
In some traditions, however, thirteen is actually a lucky number:
- In Judaism, thirteen is the age at which a boy achieves Bar Mitzvah.
- Likewise in Judaism, the Torah states that God has 13 Attributes of Mercy (also shared in some Christian traditions).
- To Sikhs, thirteen is a special number because of the legend of Guru Nanak Dev.
- The thirteenth day following a death is the day on which a memorial feast is held for the peace of the departed soul in Hinduism.
- Thirteen has special importance in the United States because it was founded with the thirteen original colonies and can be found used in a number of patriotic themes and articles, such as The Great Seal of the United States.
- The thirteen lunar cycles led to widespread belief in the number's importance in pagan cultures, one which diminished as Christianity spread and began to dismantle, discredit or assimilate pagan traditions. Or that the Chinese and Egyptians counted 2 feet and 5 fingers and they thought the next number will be unlucky