Why? Well, as explained on the company website, for many years his birthday was simply moved to tie in with some promotional campaign of one of the company's many licensees. Thus, the famous rodent's natal day was celebrated over the years on a variety of dates, from September to late December. Finally, in 1978 the corporation decided to go with the premiere date of Steamboat Willie, the Mouse's first onscreen appearance: November 18, 1928.
But back in 1953, September was still his birthday month, and that's the idea behind the ad above for the IGA grocery store chain with a 'Mickey Mouse Party' theme. It appeared in the Lorain Journal on September 10, 1953. (According to the Walt Disney Archives website, 1953 was celebrated as Mickey's 25th birthday.)
Anyway, the ad includes a nice illustration of Mickey atop his cake (yuck), surrounded by Goofy, Pluto Pup, Minnie Mouse, and Donald Duck Bongo the Bear.