The Sturgeon bay open is the weekend after Mothers Day every year. Last year we fished the NABC tournament over there (1 week before the Sturgeon Bay Open). The tournament was on Saturday and the Wednesday before the tournament there were still large chunks of ice floating around in the main lake.
There was a slight warming trend heading into the tournament and once the water temp hit 50 deg, it was like ringing a bell for the smallmouth to start moving up. Most of the top finishers were throwing a a-rigs into 2-3 feet of water focusing on the hard bottom areas of the bays. The tournament was won with a 5 fish limit that weighted 30.00 lbs.
Depending on the winter and spring weather, I would guess you could find smallmouths on beds anytime between mid-may and mid-june. But there are tons of giants swimming around those waters.
At the weigh-in for the NABC tournament, you could see several muskies cruising around the docks (water temp was 47 in Little Sturgeon at weigh in) that were over 50".