The Mid Autumn Festival or Festival moon cakes (Trung Thu) is also known as the Moon Festival, and is a popular Chinese celebration dating back over 3,000 years to China’s Zhou Dynasty. The Festival falls on the 15th day of the 8th lunar month of the Chinese calendar. The traditional food of this festival is the mooncake, of which there are many different varieties. Cakes usually have at least one dried, salted egg yolk and come with fillings of coconut, yellow or green bean, lotus seed or minced pork. All are sweet.
The Mid Autumn Festival is one of the most important holidays in the Chinese calendar. Farmers celebrate the end of the summer harvesting season on this date. Brightly lit lanterns are often carried around by children.
The Vietnamese version of this holiday is said to have originated in the 8th century, during the reign of Emperor Minh-Hoang. Legend says that the Emperor took his Empress, Duong Quy Phi, to a lake where he read a poem that he had composed to her by the light of the moon.
Source : http://www.sunshine-beach.com/news/the-mid-autumn-festival-2010/