I completely agree with wizi. Take the test when you are ready. Retake should be a last resort, not a casual plan B. BTW I am in no way insinuating that you have this attitude.
Technically, August isn't late since Applications are filed in June. Like wizi said though you'll be at a disadvantage if your scores aren't good enough. May is quite far away so I am sure you'll do well if you're planning so many months in advance (even if you are in school right now or are working full-time). In summary, don't worry about August, just worry about may and destroy the exam in one go!
I actually could have studied from July and August and taken the test in September but I knew it wouldn't be wise to apply this cycle so late. I decided to properly prepare for 3 months instead of two and just try my best in December and I'll be applying next cycle.