As for reservations, not having them should not be a problem most of the time, except for Friday and Saturday nights in the most popular areas. That being said you should always be able to find something.
There is a long weekend (1st Monday in August is always a holiday) in all three provinces. The nights of August 2nd 3rd, and 4th would require reservations. That weekend will be booked solid at most campgrounds.
Monday July 1 is a holiday in all of Canada and that weekend will be busy as well in most campgrounds.
Welcome to the Maritimes!!