The third Monday in September is a national holiday celebrating the elderly. This holiday was established in 1966 and used to be celebrated on September 15. It was moved to the third Monday of September in 2003. Prior to 1966, this was still a holiday, though not officially, Toshiyori no Hi (Old folks day) since 1951. It supposedly was first celebrated in 1947 in Hyōgo Prefecture and later spread.
This is a relatively new holiday and has very little specific customs. The point of the day is to celebrate people living a long time. Things done on this day includes: praying for elderly peoples health, calling/visiting an elderly person, talking about and thinking about welfare issues confronting the elderly.
In Tokyo, elementary school students make handicrafts for the elderly and make trips to nursing homes. The government has a ceremony and sponsors events, like visiting centenarians and giving them a gift. The Japanese are a long living race and there are a lot of centenarians, 36,276 people are over 100 in Japan this year according to Japans Health and Welfare Ministry. 86% of them are female.
As time passes this holiday will become increasingly more important. The reason for this is according to U.N. projections, Japan's centenarian population is expected to reach nearly 1 million by 2050.