"What a Diff'rence a Day Made" is a popular song originally written in Spanish by María Grever, a Mexican songwriter, in 1934 with the title, "Cuando vuelva a tu lado" ("When I Return to Your Side"). The song is also known in English as “What a Diff'rence a Day Makes”, as popularized by Dinah Washington.

The English lyrics were written by Stanley Adams, and was played by Harry Roy & his Orchestra. It was published in late 1934. The most successful early recording, in 1934, was by the Dorsey Brothers, although it was first recorded in English by Cleveland crooner Jimmie Ague.

Dinah Washington won a Grammy Award in 1959 for Best Rhythm and Blues Performance with this song. Her version was also inducted into the Grammy Hall of Fame in 1998. It also earned her first top ten Pop hit, reaching #8 on the Billboard Hot 100.

In 1975, Esther Phillips recorded her version of the song. Her comeback record had a disco feel to it. The Esther Phillips version reached number two on the disco charts. Her version also did well on the US soul and Top 40 charts. Phillips performed the song on Saturday Night Live, during its first season.

The song, as performed by Dinah Washington, is used in the soundtrack of the following films:

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