"Good-bye Days" (stylized as "Good-bye days") is a 2006 song by Japanese singer Yui, credited as Yui for Kaoru Amane. The song was released as Yui's fifth single on 14 June 2006 under Sony Records. The song served as the theme song to the 2006 film A Song to the Sun, in which Yui also starred as Kaoru Amane. The single's B-side, "Skyline", was also featured in the film as an insert song. "It's Happy Line" is actually the title track of her first single, which was an indie release.