重陽（choyo）： The Double Ninth Festival

September 9 is known as choyo (重陽), literally double yang. Choyo is one of gosekku (五節句), the five seasonal festivals and also called as kiku no sekku (菊の節句), Chrysanthemum Festival.
According to the yin and yang thought, odd numbers are yang, and its extremity is nine. Therefore September 9 is 9.9 and double yang, double nine.
People used to decorate chrysanthemum and drink sake floating the petal of chrysanthemum to celebrate wishing the long life.
Other seasonal festivals are jinjitsu (人日) on January 7th, alias nanakusa (七草), jyoshi (上巳) on March 3, alias momo no sekku (桃の節句), tango (端午) on May 5, alias shobu no sekku (菖蒲の節句), and shichiseki (七夕) on July 7, alias tanabata (七夕).