According to the I Ching, nine is a yang number, ninth day of the ninth lunar month or double nine, has too much yang and is thus a potentially dangerous date. Hence, the day is also called "Double Yang Festival" (重陽節). To protect against the danger, it is customary to climb a high mountain, drink chrysanthemum wine, and wear a plant named zhuyi (茱萸). Both chrysanthemum and zhuyi are considered to have cleansing qualities and are used to air out houses and cure illnesses.
Double Ninth may have started out as a day to drive away danger, but, like the Chinese New Year, over time it became a day of celebration. Today it is an occasion for hiking and chrysanthemum appreciation. Stores sell rice cakes (糕 a homonym for height 高) inserted with mini colorful flags to represent zhuyi. Most people drink chrysanthumum tea, a few old schools drink homemade chrysanthemum wine. School children learn poems about chrysanthemum, and many cities will host a chrysanthmum exhibit. Mountain climbing races are also popular, winners get to wear a wreath made of zhuyi.
In Cantonese culture, Double Nine is the day to visit ancestral graves. Ching Ming Festival is the traditional grave visiting day in general Chinese culture.
This is an often quoted poem about this holiday: