For big air, it's best to go in the Spring or Fall. My favorite months are late April, May, September, October and early November. Remember the you will likely need at least a 3/2 wetsuit most the time. A 5/4 full set is recommend for the colder months.
The typical wind direction comes from the North during these months and the wind is typically cold making is denser than warm wind. Plenty of days with 30+ knots and that equal to amazing jumping conditions.
My favorite spot is Waukegan on a strong North East. Great kickers for jumping regular-footed and has a marvelous flat water spot for more experienced riders.
My second and third favorite spot is South Haven and Grand Haven. They are both very similar and can have slightly different wind strength so definitely check the meters before choosing one. They both are great on a North West wind. Great kickers for goofy riders the waves sometimes get Cape Town sized on big wind days. On these North days, the jetties will form a nice fun zone flat water spot for the more experienced riders. Also a good spot for South West and the occasional Westerly winds. Plenty of beach goers here that will be taking your picture if you're going huge!
Sheboygan and Muskegon are also famous spots. They have a great thermal but I never went there for big air as the other spots are closer and typically have the same big winds.
Michigan city is great for massive waves on a big north wind day if you like boosting in huge waves. These waves get to be as big as Cape Town waves!
All the local kiters are super cool and they have a nice community. Try to link up with Stoke Riders if you can. They have an awesome kite van and are always scoping the best spots on the lake!