The Double Ninth Festival, which originated from the Warring States Period, is a tradicional festive day. The classification of all things in ancient China based on Yin-Yang Theory that is an ancient natural philosophy. Yin represents darkness and Yang represents light and power. The odd number is Yang and the even number is Yin. So according to the traditional Chinese spiritual concept, nine is a yang number; the ninth day of the ninth lunar month (or double nine) has too much yang. Hence the day is also called "Double Yang Festival"