This just in from Jim Korkis:

[In 1939, the Disney Company produced a special commercial color short for the National Biscuit Company (Nabisco) with Mickey Mouse, Minnie Mouse, Pluto and Fifi entitled "Mickey's Surprise Party". In New York for the World's Fair, Nabisco built an air-conditioned motion picture theater seating approximately 266 persons where guests to the Fair could see the short for free. For the Exposition in San Francisco on Treasure Island, Nabisco had a 130 seat theater showing the short.

An unedited version of the short is hidden on the Walt Disney Treasures: Mickey Mouse in Living Color DVD. On the second disc, press the up arrow on your remote when 'play all' is highlighted. Mickey's head appears in the "o" of Mouse. Press enter to see the cartoon. Here is a terrific collectible I recently located that is a metered envelope available at the Fair that features an advertisement for that very rare commercial short. ]