Aim for around September boys .......I was up there two years ago and mid September was just great. Let all the other travellers go fight for space in June and July. Apparently it is a madhouse up there during that time. When we went in mid September it was nice and quiet ........space to camp at any place you choose , the roads were still in relatively good condition, there was still enough water in the rivers to makes things interesting.
You cannot help but have a ball up there. Happy planning