Technically the earlier you can take it the better it is, but I would recommend taking it when you are ready. Generally anytime before the first or second week of august you'll be on a good timeline. Don't wait till august though, and definitely don't take it if you don't feel ready. If so you are just going to waste a bunch of money, and another year of your time, all because of a low score which could've been better if you studied for one or two more weeks.