[1] The attraction was being moved from Disneyland to Walt Disney World, and a replacement was sought for the original song, "There's a Great Big Beautiful Tomorrow" (also by the Sherman Brothers).
The song was written by Richard and Robert Sherman for the Carousel of Progress, one of four attractions Walt Disney and his Imagineers developed for the 1964 New York World's Fair.
General Electric, the sponsor of the Carousel of Progress, commissioned a new theme song, one to focus more on the here-and-now than on tomorrow, and the Sherman Brothers penned "The Best Time of Your Life".
The Sherman Brothers write in their joint autobiography regarding the history of the pavilion: Three years after Walt Disney World opened, the Carousel of Progress moved east from Disneyland to Florida, and we were invited to write a brand new theme song: "The Best Time Of Your Life".
"[2]As recently as February 2024, an instrumental, electronic version of "The Best Time of Your Life" can be heard around Tomorrowland in the Magic Kingdom in Florida.