Mid-July to Mid-August - Great time for the North Channel segment. Plenty of time to go as slow as you like here, it is the pinnacle of most Looper experiences.
Mid-August - About the latest for those who took the Rideau or the Champlain routes to enter the Trent Severn System.
Late August to Early September - About time to transit back into USA . Consider great detour to the Soo Locks.