Im not really sure on this, but if you order, then maybe April-May since later in summer more breeds will be sold out. But anytime in summer if not picky on breeds will be ok.
Order in Jan, Feb, or early March. I ordered 24 chicks and 5 different breeds in late April and I will not receive them till July 19th. Some of the chicks I ordered would not be ready until then. I was offered the option to ship early with substitutions but I opted not to I really wanted the breeds I ordered. So the earlier you order the better chance of you getting the chicks you want and shipped quickly.
Good luck and have fun.
I actually just ordered them the day before yesterday with mypetchicken.com . We called the place to make sure they had the chicks we wanted and placed the order on the phone. They were wonderfully helpful and honest.