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 festive happy 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（茱萸）.

It is hard to say when these customs were created. But there are many stories which are closely related. The book Xu Qi Xie Ji ,written by Wu Jun in the sixth century has one such story. In ancient times, there lived a man named Huan Jing. He was learning the magic arts from Fei Changfang, who had become an immortal（长生不老的人） after many years of practicing Taoism. One day, the two were climbing a mountain. Fei Changfang suddenly stopped and looked very upset. He told Huan Jing, on the ninth day of the ninth month, disaster will come to your hometown. You must go home immediately. Remember to make a red bag for each one of your family members and put a spray of dogwood in every one. Then you must all tie your bags to your arms, leave home quickly and climb to the top of a mountain. Most importantly, you must all drink some chrysanthemum（菊花） wine. Only by doing so can your family avoid this disaster.

On hearing this, Huan Jing rushed home and asked his family to do exactly as his teacher said. The whole family climbed a nearby mountain and did not return until the evening. When they got back home, they found all their animals dead, including chickens, sheep,dogs and even the powerful ox. Later Huan Jing told his teacher, Fei Changfang, about this. Fei said the poultry and livestock died in place of Huan Jing's family, who escaped disaster by following his instructions. And so it happened that climbing a mountain, carring a spray of dogwood and drinking chrysanthemum wine became the traditional activities of the Chongyang Festival.

The dogwood is a plant with a strong fragrance（香味）, and is often used as a Chinese herbal medicine（中药）. People in ancient times believed it could drive away evil spirits and prevent one from getting a chill in 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.

Even though the tradition of carrying a few sprays of dogwood dies out, that of climbing mountains is reaching new heights.

Early in the Western Han Dynasty, about 2,000 years ago, people used to climb a high platform outside the capital city of Chang'an on the occasion of the Chongyang Festival. For many, it was the last outing of the year before the onset of winter. The custom evolved into its present form, when people go climbing to get some exercise as well as enjoy the autumn scenery.

But what about those people who live in flat regions far from any mountain?The problem is solved by going for a picnic and eating cakes. The Chinese word for cake is Gao, a homonym（同音异义词） of the Chinese word for high. Mountains are high, so eating cake can, by a stretch of the imagination, take the place of going for a climb.

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.