The story
From man (“adult male servant”) + Friday, coined by the English writer Daniel Defoe (c. 1660 – 1731) in his novel Robinson Crusoe (1719) as the name of the protagonist Crusoe’s companion and servant; he is rescued by Crusoe from cannibal captives on a Friday. Crusoe frequently refers to him as “my man Friday”.
Etymology from Wiktionary, CC BY-SA 4.0.