Here, in Denver, early September is usually pretty nice. I know summer is winding down when the nights get just a bit cooler, and I can hear the tree crickets (or something) make that rhythmic, electric hum in the trees. They only make that noise for a brief couple weeks, and then they are gone.
We used to get an inch of snow in Denver in September, on average. But that seems to be weather of antiquity: we haven’t had a measurable September snowfall in Denver going on 17 years.
So these days September feels like extended, nicer summer: not as hot, but plenty warm. Although it was 97°F yesterday, and the lab full of 3rd–6th graders opens back up tomorrow.
So I guess summer is already gone.