Friday is one of the main characters of Daniel Defoe's 1719 novel Robinson Crusoe and its sequel The Farther Adventures of Robinson Crusoe. Robinson Crusoe names the man Friday, with whom he cannot at first communicate, because they first meet on that day. The character is the source of the expression "Man Friday", used to describe a male personal assistant or servant, especially one who is particularly competent or loyal.

Robinson Crusoe is an English adventure novel by Daniel Defoe, first published on 25 April 1719. Written with a combination of Epistolary, confessional, and didactic forms, the book follows the title character after he is cast away and spends 28 years on a remote tropical desert island near the coasts of Venezuela and Trinidad, encountering cannibals, captives, and mutineers before being rescued. The story has been thought to be based on the life of Alexander Selkirk, a Scottish castaway who lived for four years on a Pacific island called "Más a Tierra" which was renamed Robinson Crusoe Island in 1966. Pedro Serrano is another real-life castaway whose story might have inspired the novel.