The 9th day of the 9th lunar month is the traditional chongyang festival, or double ninth festival. it usually falls in october in the gregorian calendar.
In an ancient and mysterious book yi jing, or the book of changes, number "6" was thought to be of yin character, meaning feminine or negative, while number "9" was thought to be yang, meaning masculine or positive. so the number nine in both month and day create the double ninth festival, or chongyang festival.
Chong in chinese means "double." also, as double ninth was pronounced the same as the word to signify "forever", both are "jiu jiu," the chinese ancestors considered it an auspicious day worth celebration. that's why ancient chinese began to celebrate this festival long time ago.