But believe it or not, November 18, 1928 has not always been the recognized birthday of Mickey Mouse. In the recent book Mickey Mouse Treasures, author and Disney Archives manager Robert Tieman explains:
Walt Disney always said that Mickey's birthday was the day Steamboat Willie premiered, but the exact date had been lost to history. For the early birthday commemorations, the studio routinely picked a convenient day in the fall—generally a Saturday so movie theaters could schedule kiddie matinees, and generally in September (even though Steamboat Willie was nowhere near finished that early in 1928). Over the years, September 28 became known as Mickey's birthday, even making it into print in several reference books—until the establishment of the Walt Disney Archives in 1970.
One of the first things archivist Dave Smith tracked down was a vintage program from New York's Colony Theatre, the site of Mickey's debut. Inside was conclusive documentation at last of the debut of Steamboat Willie on November 18, 1928. This discovery allowed for Mickey's actual birthday to be recognized for his golden anniversary in 1978.