The very peak of hatching is usually the very end of May and first week of June. But there are a handful of early birds hatching out May 1st (or very rarely end of April), and several that don't hatch until late June.
One other thing to consider is timing of incubation when bushhogging. Hens that have just started their 28 days of incubation will readily abandon their nest as the tractor approaches, and often renest if it's destroyed. Hens closer to hatch date are not going to abandon their nest for anything and get chopped up. Because of this, if you must bushhog in the spring, do it early May rather than late May. You will chop up nests, but usually not the hen herself in early May.