That's getting towards the end of the season but a slow day on the Dean is better than most days anywhere. I've been several times and we often end up taking out at Victoria which is just above the falls. That stretch is often accessed by lower dean clients and fishes well even at the end of September. Be ready for some high water events. Weather can raise the river and put it out anytime after labor day or even before. Bring a two hander if you can. The first time I went was the first time I used one (a 14' 9wt St Croix) and I had a blast. Kind of a meat stick but the fish there have more energy than you can imagine especially in the lower waters. The end of Sept can be getting pretty cold up there but the afternoons can be sunny and warm. We may be up there around Labor Day, haven't decided if I can afford it yet.