We get our in the spring, usually around the middle of April so we don't have to heat the brooders to much. We keep the brooders in the house but it's still messy.
Since about 4 weeks is Ideal to get them out side, I guess any time so that 4th week it's not to cold at night. Maybe April through August would not be to difficult, as far as to much extra heating you would have to supply.
In the winter we try to keep the coop at least 45 degrees, which would be to hard on young birds.
As far as meat birds If they are Cornish X the July August temps in the mid west may be to stressfully. Since it can get to 100 Degrees or more. Out here I would start them earlier or later. A good time may be April to May, then September or October for fall butchering.
Tom