The Fukagawa Hachiman Festival is one of three major festival of Edo (the other two being the Kanda Matsuri and the Sanno Matsuri). It is held around mid-August and is particularly famous for the scenes of water being thrown at the mikoshi carriers by visitors, leading to the festival being known more informally as the Mizukake Matsuri or “Water Splashing Festival”. Every three years, the festival becomes the “hon-matsuri” when there is an added procession of the Hachiman imperial carriage along the streets of Koto Ward accompanied by 120 smaller mikoshi. The last one of these was in 2017.
The festival takes place over five days, but Sunday is the day of the main parade (神輿連合渡御)—an 8km route which loops the shrine to the north from 7:30am to 3:30pm. The water-splashing starts once the procession has crossed Eitai Bridge about 1km northwest of Tomioka Hachiman Shrine (link to the route from the official website). The procession departs from Tomioka Hachiman Shrine at 7:30am and proceeds anti-clockwise with the vanguard crossing Eitai Bridge around midday. As you can probably imagine, Eitai Dori from the east side of the bridge to the shrine will be extremely crowded and those with cameras are advised to take care!