The Legend about Moon
There is a mystical folktale behind moon cake, which Chinese people enjoy recounting. The folktale says a goddess in the Moon named Chang-er who offered moon cakes to the earth, it symbolize the reunion .
The legends surrounding Chang-er are prodigious. Some says she married to the celestial archer Hou Yi who saved earth from sun burning by shooting nine suns down from the air( In very ancient legend, there are nine suns in the sky, the hot tortures the lives on the earth). The Queen Mother of the West rewarded him with the elixir of life, but he was a tyrant, so to keep him away from becoming immortal, Chang-er stole the potion and drank it herself ,then, flew to the moon and has lived there alone ever since in the company of a furry rabbit. People on the earth appreciated her for stole the potion from tyrant. However, after she flew in the moon, but never can be came out. The moon isolated her from family, relatives and friends. 
People on the earth pity her very much and wish her can reunion with family someday. The legend also says on the day of 15 AUG, the moon becomes round like opened door for her only chance to escape from moon every year.
This legend symbolized people who far away from home, and wish for a reunion. By the time of 15 AUG, that remind the drifted people how much they’re missing family.
since the time to the Song Dynasty (960