Mid-Autumn Festival is a harvest festival, celebrated in most East Asian countries, such as China and Vietnam. The festival takes places on month 8 day 15 of the Chinese calendar. In 2017 it's on October 4th.
It is the second most important festival in China after Chinese New Year. To the Chinese, the festival means family reunion and peace.
It is also called "the moon festival" as it is celebrated when the moon is believed to be the biggest and fullest.