My grandfather used to say bullheads will really turn on when the first apple tree blossoms bloom!
Probably the middle of may you can get them anywhere cause i have been told this is when they start to spawn. The rivers are loaded then. Earlier they are harder to catch because they are a warm water fish that doesnt move out of the mud much in cold water. I did manage to catch one through the ice a few years ago while night fishing for walleye. Boy was I suprised.