Usually people eat moon cakes on Mid-Autumn Festival, and the round cake is symbolic of the moon in the sky and reunion in the mortal world. jieri.biz
The cake was named moon cake (yuebing in Chinese ) as early as Song dynasty (960--1279). It was given the name for its round shape, not specially designated for the Mid-Autumn Festival. In Ming dynasty the moon cake became an offering at the moon-worshiping ceremony and there was a story about lt. wish.1155815.com
Toward the end of Yuan dynasty (1206--1368) the peasants south of the Yangtze River were planning an armed uprising against the Yuan rulers. When they discussed how to pass around the message without being discovered by the officials, someone struck on a brilliant idea. They made a lot of moon cakes as offerings to be used at the moon-worshiping ceremonies and sent them to all the households concerned. On Mid-Autumn night when the people who had received moon cakes broke them, they found that the cakes all contained slips of paper inside with a written message on them saying “Every household get ready to kill the Yuan soldiers on Mid-Autumn night”. The peasants were well organized and the uprising succeeded and the Yuan government was overthrown. When Ming dynasty was established a decree was issued in memory of the uprising that moon cakes alone were designated as the food to celebrate Mid Autumn Festival. For this festival mooncakes every household would bake moon cakes to be presented to relatives and friends as Mid-Autumn gifts. This custom had to do with the way the uprising message was delivered among the peasants. In Qing dynasty there were professional shops for making moon cakes and the cakes made there were various in size, the largest ones one or two feet in diameter, and in external designs, for example Chang' e and Jade rabbits, both fairy ~mages related with the moon. 1155815
Today there are many new moon cakes and the quality and designs of moon cakes are much more refined, keeping traditional flavor. The famous ones are made in Beijing, Guangdong, Jiangsu and Yunnan. For the coatings there are soft ones, hard ones and crisp ones. For the contents there are peanuts, walnut, sunflower seeds, apricot kernel and sesame, date paste, soya bean paste and ham, etc.