My wife is Canadian, and we always visit in late summer over Labor Day. For Provincial Parks - which are mostly very nice! - they get very busy in season, especially over Holiday weekends, so we always reserve. Privately owned parks are a mixed bag - many of the most desirable ones accept seasonal rentals, and are full all summer long, and only have a few transient sites which go fast. There are parks along the major highways, etc that cater to transients, and more likely to be open for a night or two without reservations.
