Watch out Friday the 13th around the corner...

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.

The fear of Friday the 13th is called friggatriskaidekaphobia (Frigga being the name of the Norse goddess for whom "Friday" is named and triskaidekaphobia meaning fear of the number thirteen), or paraskevidekatriaphobia a concatenation of the Greek words Paraskeví (Παρασκευή, meaning "Friday"), and dekatreís (δεκατρείς, meaning "thirteen") attached to phobía (φοβία, from phóbos, φόβος, meaning "fear"). The latter word was derived in 1911  and first appeared in a mainstream source in 1953.

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, the 12 Descendants of Muhammad Imams, 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