I ordered a spring batch of Freedom Rangers and a fall batch of Cornish Cross. The FR's did take 12wks to grow out well. The CX we grew to 10wks due to circumstances beyond our control. So take a look at your life plans (vacations, visitors, sports commitments, kids commitments, parents commitments, conferences, work commitments, etc.) and calculate the best week or week and a half for you to spend time processing. Work backwards based on the breed you're hoping to raise. That's when you should order them.
A few additional thoughts are below.
The FR's were very very well insulated for feathers - wasn't very helpful once the weather turned from cool to insta-hot thanks to droughts last year. Never lost one to heat - but spent upwards of 2hrs a day filling waterers the last 3wks of their lives. (Arrived May 16, went to Camp Frigidaire mid July). Processing in the heat wasn't wonderful, but having the cool water to rinse from the hose was great to cool us off! I did add two box fans to provide air movement when Mother Nature didn't. Seemed to help them a lot. Easy to provide. Because of all their feathers, we are STILL finding their feathers in the pens, 10mo out. Loads of feathers.
The CX were remarkably hearty for being in the cool, wet fall we had. While not necessarily very well insulated in feathers, their combined body heat kept them warm despite a severe outbreak of cold weather early. (Arrived end of August, went to Camp Frigidaire first of November). While I didn't spend so much time filling waterers, I did spend a fair amount of time contending with weather-related elements...adding tarps to their tractor to provide a draft-free sleeping area when the temps took a sudden nose-dive the week before Halloween...adding plywood and 2x4 lean-to's to provide dry areas to sit in the yard...adding MORE tarps to provide rain-free spaces. Gee, I guess it rained a lot in October! Processing in the cold was awful - the concrete floor drew the cold right up your legs. Standing on area rugs helped, but mercy, it was cold! And the warm body of the chicken was hard to deal with emotionally (steam rose from their cavity as we worked on them). The hose froze in the midst of our processing, so we had to wait for temps to rise the following day to thaw out the hose! Don't recommend doing that!
I opted out of a summer batch. Glad for it. Was a brutal summer. Hoping for a better one this year! (If I wasn't watering chickens, I was watering the garden! Spent a lot of time worrying about what needed water last year.)
As for your locale - think about being outside for upwards of 2hrs a day taking care of critters...and when do YOU wish to be outside? For if you don't wish to be out there with them, then you'll likely not enjoy their presence. Good luck!