The Fukagawa Festival is officially one of the big three Shinto festivals of Tokyo along with the Sanno Matsuri and the Kanda Matsuri.
The festival generally happens on the 3 days before and after August 15th, but the actual schedule may vary slightly from year to year. The main parade starts from 7:30 am on August 12.
It’s at its rowdiest when it’s a “hon matsuri” year. The Hon Matsuri happens every three years, the last one was last year (2017).
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.