Day One
Day One may refer to:

Film and television
Day One (1989 film), a 1989 television film
Day One, also known as To Write Love on Her Arms, a 2012 drama film
Day One (2015 film), a 2015 short film
Day One (TV series), a 2009 American television pilot for NBC which was reduced from a television series to a television film, but which never aired
Day One (TV program), an American news magazine program that ran from 1993 to 1995 on ABC
"Day One" (Torchwood), a 2006 episode of the science-fiction television series Torchwood
The first episode of the 2009 Torchwood serial Children of Earth
Day One, the first season of the television series 24
Day One, the second episode of the first season of the medical drama ER
WWE Day 1, a 2022 professional wrestling event

Music
Day One (band), an English trip hop band
Day One (Birds of Tokyo album), a 2007 album by Birds of Tokyo
Day One (From Ashes To New album), a 2016 album by From Ashes To New
Day One (Sarah Slean album), a 2004 album by Sarah Slean
Day One (Snob Scrilla album), a 2009 album by Snob Scrilla
Day 1 (album), a 1991 album by Robbie Nevil
"Day 1" (song), a song by Leslie Grace
"Day One", a 2015 song by Chris Brown from Royalty

Video games
Half-Life: Day One, a Half-Life demo
Day One: Garry's Incident, a video game by Wild Games Studio
Day 1 Studios, an American videogame developer

Other
Day One (app), a personal journaling app for iOS, macOS and Android
Day One (building), part of Amazon.com's downtown Seattle corporate headquarters
Day One Christian Ministries, an organisation promoting Sunday as a day of rest, formerly known as the Lord's Day Observation Society (LDOS)