Gion Matsuri is a Shinto ritual of Yasaka Shrine with a tradition of more than 1150 years. It was first conducted as part of a purification ritual called “Gion Goryo-e” to fight off the epidemic that plagued Kyoto in 869, and then that spread throughout Japan. The ceremony was held in the Shinsen-en garden where 66 spears, representing the 66 territories of Japan during that time, were set up. Various Shinto rituals and events were held from the Kippuiri (opening ceremony of the festival in each participating neighbourhood) on July 1 to the Ekijinja Nagoshi Matsuri (closing service at Eki Shrine) on July 31. Originally, there were a pre-festival parade (Saki Matsuri) on July 17 to coincide with the temporary transfer of a shintai away from its main shrine (shinkousai) and an post festival parade (Ato Matsuri) on July 24 to coincide with the return of a shintai to its main shrine (kankousai), but in 1966 (Shōwa 41), the two grand parades were united as one. In 2014, when the "Ofune Hoko" float which traditionally brought up the rear of the procession was revived after a hiatus of 150 years, the two grand parades were separated into two parts and the festival was restored to its former glory.
The Saki Matsuri
July 10: Building of the Yamaboko floats
July 14 to 16: Yoiyama Matsuri
July 17: Yamaboko-junkō parade (Float Procession)
The Ato Matsuri
July 17: Building of the Yamaboko floats
July 21 to 23: Yoiyama Matsuri
July 24: Yamaboko-junkō parade (Float Procession)
You can also check the Gion Matsuri Yamaboko Association website.