What is the temperature like mid/late September?

The temperatures in Calgary in mid September can be very warm (26C = 79F) to really chilly (around freezing) or anything in-between, often all on the same day!!
Nights are very cool, days can be very pleasant, it is usually sunny and bright.(But beware, it isn't unusual for it to snow, although this snow won't stay around very long - a day or two).