Honestly, I'd put off a long-term vacation until the summer before you matriculate. If you really want to go this year, then aim for the last few weeks of August.
You're planning a May MCAT, but almost half of the pre-meds I know (including myself) ended up postponing/rescheduling the MCAT for a later date because of various reasons. Really try hard so this doesn't happen to you. Even if you take it in May, you should leave yourself a little bit of a cushion in case you have to re-take. This way you could get your scores back and retake the MCAT in early July and still apply this upcoming cycle.
Get your AMCAS in on day one. I know some people who submitted on the second day and weren't verified until 3 weeks later. Most schools won't send you secondaries until you're verified. I submitted the second week of July and wasn't verified until Labor Day. The faster you submit, the faster you'll be verified and get secondaries (usually 95% of them will be received within a week of your primary being verified). After you get your secondaries, it's just a matter of how fast you can churn them out. Go on this year's Allopathic school-specific threads to get the secondary essays. They don't change much from year to year. This way you can pre-write them as you're waiting for your MCAT score/AMCAS verification.
Assuming you do all this, you can definitely finish everything by August and go on vacation. Otherwise you'll burn up the whole summer with this god awful process like I did