The lyric “Summer lovin’, happened so...” is completed by “fast” in “Summer Nights” from Grease.
The song was written by Jim Jacobs and Warren Casey for the stage musical Grease. It appears in both the 1978 film adaptation and the original musical, where Danny and Sandy separately describe their summer romance to friends. John Travolta and Olivia Newton-John perform the film version with the T-Birds and Pink Ladies contributing responses.
Its structure is based on alternating viewpoints and call-and-response sections. Danny’s account is boastful, while Sandy’s version is more romantic, creating comic contrast between the two characters.
The song became a major international hit and is one of the most recognizable numbers from Grease. The phrase “happened so fast” summarizes the brief, intense summer relationship that drives the song’s setup.