The festival generally happens around August 15th, but the actual schedule may vary slightly from year to year. The full schedule is available here (in Japanese) and includes music performances, parades and Noh Theater. The water throwing section is slated to start at 9am on the 11th.
It’s at its rowdiest when it’s a “hon matsuri” year. The Hon Matsuri happens every three years. The next hon matsuri will be held in 2023. As of summer 2021, the 2021 event has been canceled and organizers state that the next event they will organize will be the 2023 event, so it may not take place in 2022 either.
The festival is centered around Tomioka Hachiman Shrine (also referred to as Fukagawa Shrine). The closest stations are Monzennakacho on the Oedo and Tozai lines and Kiba Station on the Tozai Line.