Also known as Daimonji is the end of the O-Bon festival where the spirits of the ancesors come back. This part of the festival is thought to be the signal to send the spirits back after their visit.
The fires are tended by families and passed down by tradition. The video below is a bit long but gives some background information and you get to see the burning fires.
Some further resources to look at:
http://www.kyoto-np.co.jp/kp/english/photos/gozan/gozan_index.html
http://www.kcpinternational.net/2010/08/today-in-japan-gozan-no-okuribi/
http://en.wikipedia.org/wiki/Gozan_no_Okuribi
http://gojapan.about.com/od/attractioninkyoto/a/gozanbonfirefes.htm
And don't forget to leave a comment or let me know of festivals, celebrations or holidays I am missing.