Birthday can mean:
- A celebration on a person's birthday
- Birthday's, a British greeting cards retailer
- "Birthday" (song), a song by The Beatles
- Birthday EP, an EP by Infected Mushroom
- Birthday, the fifth day of the week on the Zork calendar
- "Birthday", a song by The Sugarcubes from their 1988 album Life's Too Good
- Birthday (album), a 1968 album by The Association.
- Birthday (artwork), a 1915 surrealistic painting by Marc Chagall
- "Birthday" (Angel episode), an episode of the television series Angel
- A DiC-produced episode of the original Care Bears series
- A song by The Crüxshadows
- Birthday (The Crüxshadows EP), an EP by The Crüxshadows
B'Day, an abbreviation for "birthday", is also:
- B'Day, an album by R&B singer Beyoncé
- In Mathematics:
- Birthday paradox is a probability result that contradicts naive intuition.
- In Cryptography:
- Birthday attack is a type of cryptographic attack which exploits the mathematics behind the Birthday paradox.
