22 September is celebrated as Bilbo and Frodo Baggins’s birthday in Tolkien’s calendar.
Bilbo was born on 22 September 2890 of the Third Age, and Frodo was born on the same date in 2968. Their shared birthday is especially important because Bilbo’s eleventy-first birthday party opens The Fellowship of the Ring, while Frodo reaches adulthood in Hobbit society around the story’s beginning.
The date is observed in the real world as Hobbit Day, usually alongside Tolkien Week. It is also associated with the publication anniversary of The Hobbit, which first appeared on 21 September 1937, though that is a separate historical date. The phrase “eleventy-first” means 111th in Tolkien’s playful usage and describes Bilbo’s age at the party. The exact birthday should not be confused with the date Frodo leaves the Shire, which is 23 September in the chronology of The Lord of the Rings.