Chongyang Festival(重阳节), also called the Double Ninth Festival(九九节), falls on the ninth day of the ninth lunar month(阴历九月初九). It is named as Chongyang Festival because according to the Chinese astrology(中国古代占星术), nine is a 'Yang' character meaning masculine or positive and 'Chong' in Chinese means double. As both the month and the day are nine, it got this name. Double Ninth may have originated as a day to drive away danger.
In China, nine has the similar pronunciation(发音相似) with the word meaning 'forever', symbolizing longevity(象征长寿). Therefore, the Chinese government set the day as the Seniors' Day(老人节).
Showing respect to the seniors(尊敬长辈)
Eating Chongyang cakes（吃重阳饼）
Drinking chrysanthemum wine（喝菊花酒）
Enjoying chrysanthemum and sticking chrysanthemum,（赏菊和插菊花）
During the East Han Dynasty, there was a devil of the plague in the Ruhe River. As long as it showed up, people got sick and even died. People living around the Ruhe River suffered a lot from the disease ravaged by demons.
There lived a man named Huan Jing. His parents died from the plague and he almost died. After he recovered, Huan Jing decided to learn the magic arts and killed the devil of the plague. Huan Jing heard that there an immortal lived in the eastern area. He took a long journey and finally found the immortal. The immortal was moved by his behavior and taught Huan Jing how to kill the devil of the plague.
One day, the immortal told Huan Jing that the devil of the plague would show up again on the ninth day of the ninth month, and he asked Huan Jing to go back to kill it. When Huan Jing went back, he asked the whole family to go to a mountain and sent everyone a Zhuyu leaf and a glass of chrysanthemum wine. When the devil of the plague emerged from the water, it got dizzy from the scent of the Zhuyu and the chrysanthemum. At that time, Huan Jing took the sword and killed the devil of the plague. Since then, climbing mountains has become a custom on the Double Ninth Festival.
Double Ninth, Remembering my Shandong Brothers
By the Tang Dynasty poet Wang Wei:
As a lonely stranger in a foreign land,
At every holiday my homesickness increases.
Far away, I know my brothers have reached the peak;
They are wearing the zhuyu, but one is not present.