Some excerpts from Wikipedia:

Thirteen is regarded as an unlucky number in many cultures. Fear of the number 13 is termed triskaidekaphobia. The thirteenth of a month is likewise ominous, particularly when it falls on a Friday in some English-speaking cultures, Sweden, Russia and Germany (see Friday the 13th) or a Tuesday in the Greek and Spanish-speaking world. In Christianity: The number of participants at the Last Supper. Thirteen was once associated with the Epiphany by Christians, the child Jesus having received the Magi on his thirteenth day of life. In Judaism: 13 signifies the age at which a boy matures and becomes a Bar Mitzvah (Age of 12 for Girls, or Bat Mitzvah). The number of principles of Jewish faith according to Maimonides. According to the Torah, God has 13 Attributes of Mercy. In Mesoamerican Divination, 13 is the number of important cycles of fortune/misfortune. In Sikhism: The number 13 is considered a special number since 13 is tera in Punjabi, which also means "yours" (as in, "I am yours, O Lord"). The legend goes that when Guru Nanak Dev was taking stock of items as part of his employment with a village merchant, he counted from 1 to 13 (in Punjabi) as one does normally; and thereafter he would just repeat "tera", since all items were God's creation. The merchant confronted Guru Nanak about this, but found everything to be in order after the inventory was checked. April 13 also usually turns out to be Vaisakhi every year, which is the Sikh New Year and the major Sikh Holiday. In Persia: On the 13th day of the new year (Norouz), people consider staying at home unlucky, and go outside for a picnic in order to ward off the bad luck. Early nursery rhymes stated there were thirteen months in a year because of the natural moon cycle that was used to count the lunar year. In England, a calendar of thirteen months of 28 days each, plus one extra day, known as "a year and a day" was still in use up to Tudor times. In Tarot decks, the 13th card of the Major Arcana is Death. While Death is rarely interpreted literally, it is possible that this furthered the perception of 13 as an unlucky number. Another hypothesis about the origin of Friday the 13th as an unlucky day is attributed to this being the day that the Knights Templar were slaughtered in a collaboration between King Philip IV of France and Pope Clement V finishing with the burning at the stake of Jacques De Molay. read the article | comments