1956: Buddy Holly went to the movies. He attends a showing of the John Wayne Western The Searchers at the State Theatre in Lubbock, Texas. Twice during the film, Wayne sarcastically remarks, “That’ll be the day.”
Holly goes home and writes a song around that phrase that will lift him out of obscurity to his first taste of rock ‘n’ roll fame.
BTW - The movie's title was also the inspiration for the British Invasion band, the Searchers.