The song is described on the album as one of Morrison's classic compositions, along with "Moondance", "And it Stoned Me", "Caravan" and "Into the Mystic". The song features improvisation on Jack Schroer's alto saxophone and a backing vocal trio.
Morrison described his inspiration for the song:
"Brand New Day" expressed a lot of hope. I was in Boston and having a hard job getting myself up spiritually...Then one day this song came on the FM station and it had this particular feeling and this particular groove and it was totally fresh. It seemed to me like things were making sense....I didn't know who the hell the artist was. It turned out to be The Band. I looked up at the sky and the sun started to shine and all of a sudden the song just came through my head. I started to write it down, right from "When all the dark clouds roll away"....The song (on FM radio) was either "The Weight" or "I Shall Be Released".