Hello Christopher! Welcome to the Disney Parks Moms Panel.
As a fellow Canadian with the same options for holidays as you have posted, scheduling a Walt Disney World Resort vacation is about choosing which perceived aspects of the dates you listed are negotiable.
My family has traveled during 2 of the dates you posted (March Break and Late August) and have been unwilling to travel
during the Christmas holidays due to the high cost of flights and increased Theme Park attendance. While I enjoy traveling during March Break as the weather in Florida is pleasant and crowds are not too busy (American Spring Break is later for most schools), the deal breaker is usually the severe upcharge for flights from Canada.
That leaves the final 2 weeks of August as the time to go. While the heat is unpleasant at times, if you keep your important scheduled events to morning and night, you can avoid both the intense heat and afternoon showers. The flights from Canada are reasonable and the Theme Park crowds are lighter as many kids are back to school in America at this time.
If you
book a Walt Disney World Resort Hotel close to the Theme Parks, you can quickly return to your Resort Hotel Room if the weather becomes too unbearable.
Each time period you suggest has drawbacks but I recommend accepting the late August heat and remember that in Canada, we are only 2 months away from the bitter cold.
Be sure to visit Andy's backyard at
Toy Story Land in Disney's Hollywood Studios Theme Park whenever you decide to visit.
Good Luck!
~Todd