I killed my first bear in Ontario in June. Three of us went to my dad's cabin to hunt, and our contact did not place any bait as promised. We walked old log roads in the area and ending up taking 3 bears. This was many years ago, so I don't know what the situation is like now.
Depends what part on Ontario but snow can last into the spring season and access can limit outfitters ability to bait. Outfitters tend to only book clients for the end of the spring season. Spring weather can be cold and wet right into mid/end June
So in the fall the good outfitters baits have been active much longer than spring. And should really have more bears hitting it. September weathers usually gorgeous.