If you are looking for a traditional festival in Tokyo, check the 
Azabu Juban Festival!
The Azabu Juban Festival is held every year, and gathers many people
shutting down the whole area with food stands and shops.
Why not rent and wear a yukata to get the full experience?
This year, the festival will be held on August 22nd and 23rd.
Don't miss it!

https://www.gotokyo.org/jp/kanko/minato/event/azabujuban_nouryoumatsuri.html