We found to our chagrin that the Provincial Parks in Canada open reservations in January for the summer - we were too late to reserve good sites there for this summer, and had to make alternate plans. So - January for the Canadian parks. Private parks there, I have no idea, but July and early August would definitely be peak visiting time for the Canadian Rockies, for sure.
Yellowstone opens reservations in May for the FOLLOWING year (i.e. May 2016 to reserve summer 2017 sites). Not sure about the Tetons; we booked our site at Colter Bay RV Park (not the same as Colter Bay CG) in January this year, for this summer.
