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 an 1869 biography of Gioachino Rossini:

Rossini was surrounded to the last by admiring and affectionate friends; and if it be true that, like so many other Italians, he regarded Friday as an unlucky day, and thirteen as an unlucky number, it is remarkable that on Friday, the 13th of November, he died.

However, some folklore is passed on through oral traditions. In addition, "determining the origins of superstitions is an inexact science, at best. In fact, it's mostly guesswork." 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 signs of the zodiac, twelve hours of the clock, twelve tribes of Israel, twelve Apostles of Jesus, twelve gods of Olympus, 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.

On the other hand, another theory by author Charles Panati, one of the leading authorities on the subject of "Origins" maintains that the superstition can be traced back to ancient myth:

The actual origin of the superstition, though, appears also to be a tale in Norse mythology. Friday is named for Frigga, the free-spirited goddess of love and fertility. When Norse and Germanic tribes converted to Christianity, Frigga was banished in shame to a mountaintop and labeled a witch. It was believed that every Friday, the spiteful goddess convened a meeting with eleven other witches, plus the devil - a gathering of thirteen - and plotted ill turns of fate for the coming week. For many centuries in Scandinavia, Friday was known as "Witches' Sabbath."

Another theory about the origin of the superstition traces the event to the arrest of the legendary Knights Templar. According to one expert:

The Knights Templar were a monastic military order founded in Jerusalem in 1118 C.E., whose mission was to protect Christian pilgrims during the Crusades. Over the next two centuries, the Knights Templar became extraordinarily powerful and wealthy. Threatened by that power and eager to acquire their wealth, King Philip secretly ordered the mass arrest of all the Knights Templar in France on Friday, October 13, 1307 - Friday the 13th.

The connection between the superstition and the Knights Templar was popularized in the 2003 novel The Da Vinci Code. However, some experts think that it is relatively recent and is a modern-day invention. For example, the superstition is rarely found before the 20th century, when it became extremely common. 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.

A further theory goes back to a combination of Paganism, Christianity, and the Battle of Hastings. For many, the number 13 was considered a lucky number (such as 13 lunar cycles each year), but with the efforts of Christianity attempting to degrade all things Pagan, they promoted 13 as an unlucky number, with Friday thus also being considered a bad day of the week. However, on Friday the 13th of October 1066, the decision was made by King Harold II to go to battle on Saturday the 14th of October, rather than allow his troops a day of rest (despite his army having made a long and arduous march from a battle near York just 3 weeks earlier).

This decision in going to battle before the English troops were rested (the English lost and King Harold was killed), further established Friday the 13th as an unlucky day.

In some other countries, instead of Friday, Tuesday the 13th is considered bad luck. For example, the Fall of Constantinople, when the city fell to the Ottomans (a fact which marked the end of the Byzantine Empire), happened Tuesday, May 29, 1453, and is why Greeks consider Tuesday to be an unlucky day.

I hope it was a good day for everyone.

Pip-pip!