September 21 is the 264th day of the year (265th in leap years) in the Gregorian calendar. There are 101 days remaining.
- 2001: "Sailor Mouth" and "Artist Unknown" premiered.
- 2004: Bubble Blowers, Beware!, Ice Cream Dreams, Buried Treasure, SpongeBob Exposed!: The Insider's Guide to SpongeBob SquarePants, Trouble at the Krusty Krab!, and The SpongeBob SquarePants Movie book were published.
- 2010: The Great Train Mystery was released.
- 2017: "Feral Friends" and "Don't Wake Patrick" premiered in Iberia.
- 2018: "Krusty Kleaners" premiered in Poland.
- September 21 on Wikipedia