I am planning a September trip to Burk's Falls/Algonquin day paddling etc from September 1 until September 8. You can come for as much or as little as you like. You probably don't want to travel up on the 5th or home on the 7th as this is their long weekend (Labor Day) as well and the roads could be crowded.
There is some flexibility in these dates as I will be getting up there on the 26th (returning from Yukon Trip) but will need a few days to relax and also get ready for you all.