Of course, all Mouseketeers should know that the Mouse's recognized birthday is November 18, 1928 (the same day Steamboat Willie premiered).
If the above frame looks strange, it's probably because the animators used a mirror effect to create a larger group of animals by simply reusing animation. The technique saves time and money as well as creates two of every guest. Just behind Mickey stand two Clarabelles and two Horaces.
Notice how there are two candles on Mickey's cake. This leads me to think that perhaps this toon was originally planned for release in 1930 (Mickey's 2nd year) and for whatever reason was pushed back to the beginning of 1931. In any case, I suppose the film can be seen as a belated birthday party for the Mouse.
The cartoon ends with Mickey playing his xylophone (which seems to appear from nowhere).
Though not very special, The Birthday Party is an entertaining toon and goes on to inspire a few remakes involving not only Mickey and Minnie, but other yet to be seen Disney characters as well.