Is Sept too late for visiting Canadian Rockies?

Here is a great article on September weather in Banff. The answer is "it depends". The average daytime temperature in September for Banff is 61F (16.1C). The night-time lows average 37F (2.8C). Of course, those are averages, and you can go from one extreme to another pretty quickly. If it were me, I'd go, but be prepared to adjust your schedule in case you have to wait out a day or two of snow.

Banff Weather for September

Jim