Friday the 13th occurs when the thirteenth day of a month falls on a Friday, which superstition holds to be a day of bad luck. In the Gregorian calendar, this day occurs at least once, but at most three times a year. Any month's 13th day will fall on a Friday if the month starts on a Sunday.
Rossini by Henri Grevedon According to folklorists, there is no written evidence for a "Friday the 13th" superstition before the 19th century. The earliest known documented reference in English occurs in Henry Sutherland Edwards' 1869 biography of Gioachino Rossini:
Rossini was surrounded to the last by admiring and affectionate friends; Why Friday the 13th Is Unlucky
Consequently, several theories have been proposed about the origin of the Friday the 13th superstition.
One theory states that it is a modern amalgamation of two older superstitions: that thirteen is an unlucky number and that Friday is an unlucky day.
In numerology, the number twelve is considered the number of completeness, as reflected in the twelve months of the year, twelve hours of the clock, twelve gods of Olympus, twelve tribes of Israel, twelve Apostles of Jesus, etc., whereas the number thirteen was considered irregular, transgressing this completeness. There is also a superstition, thought by some to derive from the Last Supper or a Norse myth, that having thirteen people seated at a table will result in the death of one of the diners. Friday has been considered an unlucky day at least since the 14th century's The Canterbury Tales, and many other professions have regarded Friday as an unlucky day to undertake journeys or begin new projects. Black Friday has been associated with stock market crashes and other disasters since the 1800s. It has also been suggested that Friday has been considered an unlucky day because, according to Christian scripture and tradition, Jesus was crucified on a Friday. One author, noting that references are all but nonexistent before 1907 but frequently seen thereafter, has argued that its popularity derives from the publication that year of Thomas W. Lawson's popular novel Friday, the Thirteenth, in which an unscrupulous broker takes advantage of the superstition to create a Wall Street panic on a Friday the 13th. Records of the superstition are rarely found before the 20th century, when it became extremely common. The connection between the Friday the 13th superstition and the Knights Templar was popularized in the 2003 novel The Da Vinci Code. However, experts agree that this is a relatively recent correlation, and most likely a modern-day invention.
Friday the 13th originated from Friday being the sixth day of the week, and the number 13, both having superstitions rooted in ancient times. Friday was Ancient Rome's execution day while in Britain it was the customary Hanging Day. Accidents are also common on Fridays. See more on http://urbanlegends.about.com/cs/historical/a/friday_the_13th.htm.