The number 九 (9) in Chinese philosophy is considered to be a 陽 (Yang, masculine nature) number. The primal element complimenting this is 陰 (Yin, feminine nature) and together in symbiosis create the metaphysical concept of 陰陽 (Yin Yang).
So, the 9th day of the 9th month of the Chinese lunisolar calendar bestows us with 重陽節 (Double Yang Festival) and considered a potentially dangerous date due to the imbalance of 陽 (Yang, masculine nature). To protect against calamity, it is customary to go hiking and climb high mountains and make use of 菊花 (Chrysanthemum) and 茱萸 (Zhuyu) as both plants are considered to have cleasing qualities. 菊花 (Chrysanthemum) can be used to make tea or wine and 茱萸 (Zhuyu) is typically just worn on the person.
The festival is also known as Autumn Remembrance and is a time for families to journey to the graves of their ancestors to perform cleansing rites and pay their respects. On the way they’ll share food with one item that’ll certainly be brought along is 米糕 (rice cakes) since 糕 (cakes/pastry) is a homonym for 高 (high/tall) and therefore auspcious.
Origins for the festival points to a 漢朝 (Han Dynasty) legend where a soothsayer advised Woon King that he should take his family to a high place for the entire 9th day of the 9th month. Upon their return, the Woon family discovered all living things in their village had been slaughtered and the tradition to escape misfortune lives on in commemoration of that day!