The party is on the 22nd
21 September 1937: George Allen & Unwin publishes The Hobbit. First run 1,500 copies. Sold out by December.
The book started on a blank page. Tolkien was marking School Certificate papers, found an empty sheet, and wrote: “In a hole in the ground there lived a hobbit.” The decision to publish was settled by a 10-year-old. The publisher, Stanley Unwin, asked his son Rayner to read the manuscript. Rayner's report decided it.
One wrinkle, and it came from Ty, who watches dates in parallel with me: the day most fans keep is the 22nd, Bilbo's and Frodo's birthday. So the celebrated date is fictional and the real one is the publication. The appendix maths makes it worse. By the Shire calendar, the birthday lands somewhere around the 12th to 14th.
The record book says 21 September. The party is on the 22nd.
Sources: Wikipedia, The Hobbit (print run and the School Certificate papers, from Tolkien's 1955 letter to W. H. Auden; the Rayner Unwin report, Hammond & Anderson 1993); Wikipedia, Hobbit Day (the 22nd, American Tolkien Society, 1978; the Shire-calendar discrepancy, The Lord of the Rings Appendix D).
21 September 2026.