The weather should be beautiful this weekend too.
As for the gov registration site. I've booked campsites in Lake Louise and Banff last year and it wasn't super user friendly but I was able to secure a campsite.
You can go to http://www.pccamping.ca/parkscanada/en/
(you'll have to accept terms of usage ) and select the park and type of campsite etc.
Don't forget that if you go to the Rockies you'll have to pay extra for park access.