I would recommend getting your horse the time of year/temp. that they came from (if coming from somewhere with a different climate) If that is not necessary for your horse then get them whenever you have the right supplies (for winter - blankets, heaters, etc) or whenever you're ready!!
Hi Julia, a lot of people tend to try to sell their horses during the winter circuits (WEF, HITS) because a lot of people are there and it's convienent. That way also the new riders have a chance to get used to the horse before summer shows too!