mid-September - be prepared for snow flurries. If it snows, it likely won't stick around long but if you're not comfy towing in snow/slush you can choose to wait it out until it melts away.
One year, the second week of September, we endured an ice storm while driving through Fort Nelson. The highway had a layer of ice on it and all the vehicles had ice hanging from them. A few miles south of Fort Nelson, all was clear again.
This also means you need to be set up for camping in below freezing temperatures as it does get cool at night, especially when the skies are clear. Many campgrounds close after Labour Day, a limited few stay open year-round. Best to be prepared to camp in the cold without hookups, just in case.