Friday the 13th occurs when the thirteenth day of a month falls on a Friday, which superstition

Superstition

Superstition is a belief in supernatural causality: that one event leads to the cause of another without any process in the physical world linking the two events....

 holds to be a day of bad luck

Bad Luck

Bad Luck may refer to:* Harmful or negative luck* Bad Luck , a 1960 film directed by Andrzej Munk* "Bad Luck" * "Bad Luck" * Bad Luck, an album by Trophy Scars...

. In the Gregorian calendar

Gregorian calendar

The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

, 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 (Frigg

Frigg

Frigg is a major goddess in Norse paganism, a subset of Germanic paganism. She is said to be the wife of Odin, and is the "foremost among the goddesses" and the queen of Asgard. Frigg appears primarily in Norse mythological stories as a wife and a mother. She is also described as having the power...

a being the name of the Norse goddess for whom "Friday" is named and triskaidekaphobia

Triskaidekaphobia

Triskaidekaphobia is fear of the number ; it is a superstition and related to a specific fear of Friday the 13th, called paraskevidekatriaphobia or friggatriskaidekaphobia.The term was first used by Isador Coriat in Abnormal...

 meaning fear of the number thirteen), or paraskevidekatriaphobia a concatenation of the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 words Paraskeví (Παρασκευή, meaning "Friday"), and dekatreís (δεκατρείς, meaning "thirteen") attached to phobía (φοβία, from phóbos, φόβος, meaning "fear").