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 one will be in 2020.
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.