"Greatest Day" is a song by British singer Beverley Knight, released as the fifth single from her second studio album, Prodigal Sista (1998), on 5 July 1999.

The track contains a re-recorded sample of "Act Like You Know" by American R&B group Fat Larry's Band.

"Greatest Day" became Knight's highest-charting single up to that point when it peaked at number 14 on the UK Singles Chart.

The music video for the song was shot by director Jake Nava, who had previously shot Beverley's "Made It Back '99" video.

Personnel are taken from the UK CD1 liner notes.