Watching the John Wayne film "The Searchers" and was looking at the trivia on IMDB and came across this.
Reportedly this film was seen in a theater in Texas by Buddy Holly and his friends in the summer of 1956. They were so impressed with Ethan's (John Wayne) repeated use of the phrase "That'll be the day" that they used it as the title for their now standard rock song, which they composed soon after.
