Jade Rabbit Making Heavenly Medicine
This legend is considered to be an extension of Chang'e Flying to the Moon. It tells about three sages who transformed themselves into pitiful old men. One day they met a fox, a monkey and a rabbit, and they begged for food. The fox and the monkey shared their food with the old men; but the rabbit, with nothing to share, jumped into a blazing fire to offer his own flesh instead. The sages were so touched by the rabbit's kindness that they sent it to live in the Moon Palace, where it became the Jade Rabbit. Chang'e, who lived alone after arriving in the moon, liked the Jade Rabbit at the first sight, and therefore kept it company. As the time went by, Chang'e and Jade Rabbit became inseparable friends. Hearing about the story of Chang'e and Hou Yi, the Jade Rabbit felt sympathetic to them and decided to make a special heavenly medicine, which could help Chang'e return to Earth. Unfortunately, the Jade Rabbit has still not been able to make it even though he has worked hard for thousands of years. Observing the moon on the Mid-Autumn Day carefully, we can still see the Jade Rabbit making his heavenly medicine.
Wu Gang Chopping the Cherry Bay
There is also another story about the Festival moon cakes. Many years ago, Wu Gang, an immortal in Heaven, was punished to chop down the cherry bay in the moon for his serious mistakes. The cherry bay in the moon was extremely luxuriant and sturdy with a height of 5167 meters. It would also heal up as soon as it was chopped, which made Wu Gang's work last for thousands of years.
The mid autumn festival has many traditions and activities in which people express how much their families mean to them, and how much they miss absent members and enjoying mooncake as the symbol of the legends.
Source : http://www.chinatravel.com/facts/chinese-festivals/traditional-chinese-festival/Middle-Autumn-Festival.htm#1