According to the lunar calendar, Mid Autumn Festival is celebrated annually on the 15th day of the 8th month. (which is a different date each year).
History dates it back to the Zhou Dynasty, but there were no festivals held at that time. By 1127AD, the Northern Song Dynasty held celebrations, and since then, this custom has grown.
Depending on where you live, this festival is celebrated in different ways. One tradition however, is to gather with family (and friends), eat mooncakes (see the image below), and gaze up at the moon. If family members are far apart over this festival, looking up at the moon is said to ease the yearning.