September 9 of the lunar calendar is the traditional Chongyang Festival, which is also called “Double Nine Festival” or “Senior Citizens Day”. The Chongyang Festival is a traditional festival mingling various folk customs, and unlike other festivals that are shared by multi ethnic groups such as the Spring Festival, the Dragon Boat Festival, the Mid-Autumn Day etc, the Chongyang Festival is exclusively celebrated by the Han Chinese. According to the conception of folk custom, the number "nine" is a homophone of “久” (jiu, for a long time), thus the Double Ninth refers to “久久”, which carries the meaning of “long-lasting life” or “longevity”. After it was specified as the “Senior Citizens Day”, the festival has been focused more on the traditional virtues of honoring, respecting, loving, and helping the old.

September 9 of the lunar calendar is the traditional Chongyang Festival. As the Book of Changes specifies “six” as a number of yin and “nine” a number of yang, so September 9 has the yang number in both month and day, two "nines" meeting on this day, thus the name “Chongyang” (dual yang) or “Chongjiu” (dual nine). However, the title “Chongyang Festival” did not appear in records until the Three Kingdoms Period, when Cao Pi mentioned it in Writing to Zhong Yao on the Double Ninth Day.

The origin of the Chongyang Festival can be traced back to the Pre-Qin Dynasty. According to Master Lv's Spring and Autumn Annals, the activities of offering sacrifice to gods and ancestors in September when the crops were harvested to express the gratitude had been practiced at that time. It is said that from the Han Dynasty on, the custom of praying for longevity on the Chongyang Festival came into being. It was resulted from the influence of ancient wizard's (Taoist's) pursuit of eternal life and taking collected herbal medicine. In the meantime, the activity of large-scale banquet is evolved from the feast of celebrating a bumper harvest in the Pre-Qin Dynasty.

Source: traditions.cultural-china.com