I think it depends on your climate...or at least I'd ask a couple local vets and see what they think.
My vet likes late/early fall (depending on the frost to kill the worst of the flies/mosquitoes/what have you) and/or Spring before the bugs are an issue, but after the worst of the mud.
Ideally, he likes the ground to be fairly firm so they can be outside recovering as much as possible and still have as few bugs around as well. At least this is how he explained it to me.