The festival is based on the theory of Yin and Yang, the two opposing principles in nature. Yin is feminine, negative principle, while Yang is masculine and positive. The ancients believed that all natural phenomena could be explained by this theory. Numbers are related to this theory. Even numbers belong to Yin and odd numbers to Yang. The ninth day of the ninth lunar month is a day when the two Yang numbers meet. So it is called Chongyang. Chong means double in Chinese. Chongyang has been an important festival since ancient times.

The festival is held in the golden season of autumn, at harvest -time. The bright clear weather and the joy of bringing in the harvest make for a happy, festive atmosphere.The Double Ninth Festival is usually perfect for outdoor activities. Many people go hiking and climbing in the country, enjoying Mother Nature's final burst of color before she puts on her dull winter cloak. Some will carry a spray of dogwood.

The dogwood is a plant with a strong fragrance, and is often used as a Chinese herbal medicine. People in ancient times believed it could drive away evil spirits and prevent one from getting a chill in late autumn. So its history as a medicine goes back many centuries. But the custom of carrying a spray of dogwood during the Double Ninth Festival is slowly dying out and many people, especially young people in the cities, do not even know what a dogwood spray looks like.

Since nine is the highest odd digit, people take two of them together to signify longevity. Therefore, the ninth day of the ninth month has become a special day for people to pay their respects to the elderly and a day for the elderly to enjoy themselves. It has also been declared China's day for the elderly.