The two flattest months of the year in the Great Lakes are probably May and June. I say probably because I haven't taken the time to research historical data. This is strictly from my own experience, and some may think differently.
It's pretty simple to understand why. We need wind to make waves - a sustained minimum of 15 knots or more. Wind is typically associated with bigger weather systems like low pressure, cold fronts, and other fun meteorological phenomena. Those systems are more frequent in fall, winter, and early spring, which is why we get more surf in those seasons. They are not as frequent in May and June.
Sure, it sucks to not have surf for days or weeks at a time. It can be frustrating. Some might call it a form of torture.
But, maybe it can be a blessing in disguise, too?
The joy of hooking into a decent peak after after not surfing in what seems like forever is where its at. For better or for worse, we know that feeling pretty well here.