According to the "I Ching", "nine" is a yang number; the ninth day of the ninth lunar month (or double nine) has too much "yang" (a traditional Chinese spiritual concept) and is thus a potentially very auspicious date. Hence, the day is also called "Double Yang Festival" (). It is customary to climb a high mountain, drink chrysanthemum liquor, and wear the "zhuyu" () plant, "Cornus officinalis". (Both chrysanthemum and "zhuyu" are considered to have cleansing qualities and are used on other occasions to air out houses and cure illnesses.)