From "Managing Michigan's Wildlife: A landowner's guide"

The timing of a burn determines the plants which will be benefited and controlled, the impact on wildlife species, and safety. Most burns are conducted mid to late spring, or in the fall. Burning to favor desired grasses should take place just as they are starting to green up, and the soil surface is damp. Generally, a late spring burn will control woody vegetation and cool season grasses better than an early spring burn but are not as beneficial for wildflowers. This burn will also provide warm season grasses with nutrients they need to grow. 

Before burning, nesting times of grassland species should always be checked to prevent the destruction of nests and their inhabitants. The best time for spring fires is late March into April; generally in the morning or evening, when the relative humidity and temperature are not changing as rapidly as during daylight hours. The drier the area the earlier the burn should be to avoid damaging the earliest blooming wildflowers. Though fall burns are possible and can be beneficial, they are often avoided, due to the cooler temperatures, drier ground, and destruction to winter wildlife habitat they may cause.