I would like to know the origin of Mid-Autumn Festival

In ancient times, the emperors followed a special ritual system. In Spring, they worship the sun. In autumn, they worship the moon. Later, literatis, rich people and civilians learn from the emperors to worship the moon. During the Tang Dynasty, this ritual had been observed widely. Then they chose a special date (August 15 on Chinese lunar calendar) and set it as Mid-Autumn Festival.