Why is the number 13 supposed to be unlucky?

As lucky, unlucky, or significant number

Thirteen is regarded as an unlucky number in many cultures. One (probably unverifiable) theory holds that 13 is a reflection of the human fear of the unknown, since it is the first number that cannot be enumerated by using our 10 fingers and 2 feet. Unreasoned fear of the number 13 is termed triskaidekaphobia. Due to this fear, some tall buildings have resorted to skipping the "thirteenth floor", simply by numbering it "14" (though it's really still the thirteenth floor) or by designating the floor "12a" or similar instead. The thirteenth of a month is likewise ominous, particularly when it falls on a Friday (see Friday the 13th), or in the Spanish-speaking world, a Tuesday. The 13th falls on a Friday more often than any other day. In fact it occurs more often than any other day/number combination except those that coincide with it (Thursday the 12th, Saturday the 14th, etc). Months with a Friday the 13th always begin on a Sunday.

A small minority of left handers (see "left handed") consider 13 to be their "lucky" number. This is possibly derived from unlucky being an antonym for lucky (7 is a popular lucky number in Western culture). Many left handers are taught to do the opposite when learning new skills from a right handed instructor. Thus they consider 13 to be their "lucky" number as it is the opposite of 13 being the "unlucky" number.

The number is also considered unlucky by some as it is the sum of 1-4-5-3, the year of the fall of Constantinople to the Turks, marking the end of the Byzantine Empire, though it may be considered lucky by Turks, on the same basis!

In the Christian religion, the number is considered significant since there were thirteen present at the Last Supper. Also, if both Judas and Matthias are counted, there were thirteen apostles. Thirteen was also once associated with the Epiphany by Christians, the child Jesus having received the Magi on his thirteenth day of life.

In the Jewish tradition, 13 signifies the age of maturity, a 13 year old Jew is said to be Bar mitzvah. 13 appears in other places in the Hebrew Bible though not as often as other famous biblical numbers such as 7 and 12. Thirteen also occurs in different list of important characteristics, such as God's thirteen attributes or Maimonides Thirteen Principles of Faith. In fact, one could say that for Jews, 13 is more of a lucky number than an unlucky one.

In the Sikh religion, 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"). When Guru Nanak Dev Ji had a job of counting stocks of items, he counted from 1 to 13 (in Punjabi) as one does normally, but after reaching the count of 13, he would just repeat "tera", since all items were in fact God's, as God created them. When somebody heard this while passing, Guru Nanak Dev Ji was confronted about this, and his records were checked. It was a miracle since they were all perfect, yet he never seemed to count the items properly.

Modern day witches have reclaimed the number 13 as a lucky and significant number. This may be similar to witches reclaiming other previously negative terms and ideas associated with witchcraft, such as black cats, brooms, and even the term witch itself. In modern day Wicca, thirteen is considered the maximum size of a coven, and in some traditions the ideal number of members.

Amongst some Chinese people, it is regarded as a lucky number because it sounds similar to "hahaha23526;hahaha29983;", which means "must be alive". But this belief is not universal, or even known to all Chinese. Also, 13 is regarded as unlucky by those Chinese under the influence of foreign cultures.

In the Persian's (Iranian's) culture, 13 is also considered an unlucky number. On the 13th day of the Persian new year (Norouz), people consider staying at home unlucky, and go outside for a picnic in order to ward off the bad luck.

In The Hitchhiker's Guide to the Galaxy, "What is six times nine" is posed as the question to the answer to life, the universe and everything (42). This is true in base 13, although this was merely coincidence according to author Douglas Adams.

http://en.wikipedia.org/wiki/13_%28number%29