Traditional Activities During the Mid-Autumn Festival:
Besides the obvious, moon-gazing, Chinese families celebrate by getting together and eating. Boiled peanuts, slices of taro, rice gruel, fish and noodles are all traditional dishes to eat during the festival, but none of these takes the place of the moon cake. Ubiquitously on sale in every supermarket and hotel, moon cakes are now a highly prized commodity. Companies use the festival as a time to thank clients with boxes of moon cakes. You may be wondering just what a moon cake is?

Moon Cakes:
Moon cakes are typically round, symbolizing the full round moon of the Mid autumn festivals. They are usually made with four egg yolks, representing the four phases of the moon, and are sweet, filled with sweet bean or lotus seed paste. There are savory types as well and these days, you can even get them from Haagen Dazs. Read more about moon cakes and how to make them from Rhonda Parkinson, Guide to Chinese Cuisine.

History of the Chinese mooncake:
Based on a legend, it was with the help of the moon cake that the Ming Dynasty established. Rebels used the Mooncakes festival as a method to convey their plans for revolt. They ordered the baking of special cakes to commemorate the festival. But what the Mongol leaders didn’t know was that secret messages were tucked into the cakes and distributed to allied rebels. On the night of the festival, the rebels successfully attacked, overthrowing the Mongol government and establishing a new era, the Ming Dynasty.

Source : http://gochina.about.com/od/eventsfestivals/p/MoonFestival.htm