'Un bel di,' by Giacomo PucciniIt woud be difficult for a single tune from Madame Butterfly to be more popular than the opera itself, but there's one that comes close: Cio-cio-San's poignant aria, "Un bel di" — "One fine day."
Washington National Opera on World of Opera -- 'Madame Butterfly'
Puccini's Madame Butterfly is one of the most beloved operas of all time, so it would be difficult for any one of its many great tunes to outstrip the opera itself. But one of its arias comes close. Cio-cio-san is warned that Pinkerton, her American husband, may never return. She responds with "Un bel di," a song expressing great longing, and great faith, saying that on "One fine day," she'll gaze at Pinkerton's ship sailing into harbor.