Our trip to Banff, Jasper, etc. had to be postponed until late Sept./early Oct. Will this be too late (really cold, too many things closed). Has anyone been there at that time? Thanks a lot.

Will it be really cold? Well it could be really cold in July or it could be really hot. Our weather is that unpredictable. Be prepared for chillier weather and you'll be fine. Nothing should be closed as these are year round destinations.

it is a great time to visit. the rates are usually a little lower and less crowds. you may have hot sunny days but the nights will get chilly so you'll have to bring a variety of cloths.

Banff and Jasper in Fall is very temperate, comfortable. Golf course at Banff Springs will be closed in October (only elk roaming, although its still very green).

That time of year is a beautiful time to go. Generally, you should be pretty safe - as far as really bad weather. In terms of things being closed - it depends on what you're looking for. As mentioned earlier, golfing could be hard to find. Many of the campgrounds close in late August or obly allow "hard-sided" camping. Some the the hiking trails with high elevation could be closed as well - these are well posted at any "Friends of" (Banff, Jasper, etc.) location or back country office. Weather is always something to keep in mind. I usually go in late July or late August and still pack mittens, hat and warm coat.

Have fun!

It depends on what you are used to - what is "cold" for you?
Banff is at a high elevation (1384m or over 4000 ft above sea level), the air is thin (so for you sea level types, you may find that you are sleepier than usual for the first few days in the mountains), and thin air does not hold heat very well.
Translation = even if it is beautiful during the day it can be cold at night.
So what is cold and what is warm??
Average daily high temp for Banff for Sept is 16C (61F), for October cooler, 10C (50F). But it can range anywhere from frosty (below 0C = 32F) at night to hot (27C = 80F) during the day.
Jasper is at a lower elevation and is warmer than Banff.
Basically - be prepared for anything!!
Good news is that Sept and Oct are usually drier than the summer months, so there should be plenty of clear blue skies.
I travel with mitts and hats only in winter, but as I said earlier, it depends on what you are used to; however, if you plan on any hikes to higher elevations definitely be prepared for cold and wind and rapidly changing conditions.
Best approach is to plan on dressing in layers, so you can add or remove according to the weather conditions.
By the way, as an earlier message pointed out, Jasper and Banff are year-round destinations, so most things are always open (except for camp grounds and golf courses, the latter because elk like to use golf courses for mating in the fall and the males can get very aggressive).
Have a wonderful stay in the Rockies!!