Are we talking DO here? While timing isn't AS important as allopathic schools, it still helps a lot...so you should really be planning to get that Primary AACOMAS application in by June...and those secondaries in by the end of August.
What I was referring to by "the first time" is that 1) taking it for the first time by July ensures your application will be complete by sometime in August (assuming Primary/Secondaries/Letters of Rec are in by then...which as I just said they should be). and 2) Allows leeway for you to retake it if you're not happy with the score you got or if med schools straight up tell you "Retake the MCAT, do X points better and get back to us" (as they've told one of my friends).
Hope that clarifies things as far as the timing of this all is concerned? One thing to note is that while the primary application and secondaries list "deadlines" they're not really deadlines. Because of rolling admissions in med school (accepting people throughout the process rather than on one date), the earlier you're complete the better. If you're ready to take that MCAT sooner than July, you should. April used to be the tradition before the computerization and multiple tests a year format...May seems to be the most common now (based on my unscientific experience).