Koenji Awaodori will be held this weekend, 25th and 26th August.
Note that the English site states from 5PM - 9 PM
And the Japanese site states 5PM - 8 PM
I'd say it from 5-8 as the dancing has been always been for 3 hours in past events.
Either way, the finish time matters not, just make sure you get there around 3PM to avoid the hordes of people.
If you get there anytime around 5, you'll be caught up in the confusion of the crowds, and it does get busy in those narrow streets, and the best view you'll get 5-rows back is the top of someones head.
A lot of the regulars and Koenji residents (I'll include myself in that list) will already have claimed their spot.
It's one party that isn't cool to be "fashionably late".
First time visitors to Koenji might notice that there are two exits, north and south, but it will all be changed and roped off and as you get off the platforms, you'll be ushered out of the exit like sheep.
North or south, you say?
In a nutshell, the South has dancing taking place down the main street, which, if you get there late on, is a complete waste of time as you'll be struggling to get a decent view.
To the side of the main street is the PAL shopping arcade, which is a good spot, if you're there early enough, but it gets very hot in there.
To the north, the action takes place a few minutes away on a smaller street and also in the back streets.
If you're coming late, the north side is probably better to get a decent view.
Get there early and have a wander around as a lot of local backstreet shops will be selling some good food and overpriced beer.
Stuff your face with food beforehand and stock up on drinks.