The origins of the holiday are fairly recent.
It is said 'The Legend of Yōrō Falls' (養老の滝) inspired the mayor of Nomadani-mura (野間谷村), in Hyōgo Prefecture, to create the first "Old Folks' Day" (としよりの日 "Toshiyori-no-hi").
Let us put aside for the moment that this famous tale likely took place in October.
On September 15th 1947 Nomadani-mura (野間谷村 - later Yachiyo-chō 八千代町, currently Taka-chō 多可町八千代区) in Hyōgo Prefecture hosted the first "Old Folks' Day".
The festival soon spread nationwide and in 1966 it was declared an annual national holiday.
Japan, famously, has the oldest citizenry in the world. 28.4% of its population is 65 years or older (1 in 5 people are over 70).
September 15th was selected for this holiday because ancient records indicate that this is when Prince Shōtoku (聖徳太子 574-622) opened the Hiden-in at Shitennō-ji.
The Hiden-in looked after the sick and elderly who had no-one to care for them.
Camellia Tea Ceremony