Why must Chinese celebrate mooncake festival?

The Mooncake Festival is also known as The Lantern Festival, Mid-Autumn Festival, or in Chinese, Zhongqiu Jie (中秋节). It is probably the most important holiday for the Chinese after Chinese New Year.

Celebrated on the fifteenth day of the eighth moon of the year, it is a night for many people to sit out and gaze at the moon as it also at it's roundest and brightest. For farmers, it's their last day of the summer harvest. For most normal people it's a day they come out to worship the moon god and thank the heavens (which the moon is a representation) and earth. They will pray to the moon god for good fortune, family unity as well as protection.

There is no requirement and the Chinese do not HAVE to celebrate the Mid-Autumn Festival. They just choose to as many western society choose to celebrate Easter or Christmas.