I was in the same boat as you and had to take anatomy and physio over the summer and I ended up taking the GRE at the end of August, about one week after my summer classes finished. I did not study as much as I wanted to, so I would start studying now regardless. Read the ETS website thoroughly - they have 2 practice tests which prepare you for the real test come test day. They also have a math review packet that reviews all concepts on the quantitative portions and they have a listing of every single possible prompt on the writing sections you will have.
Some people are better test takers than others, but my advice is to know the test inside and out, at least take the 2 ETS practice tests, and prepare yourself mentally for the environment on test day. I would also practice the writing essays under the time constraints, no matter how good of a writer you think you are. I know I only wrote 1 or 2 practice essays and several of my friends didn't practice for that section at all. I walked away with a 4.5 in that section, but my friends were in the 3-3.5 range, and I would consider us all to be very good writers. I would say either take the test before your classes start or sometime in the middle of the summer. When scheduling, do be mindful of deadlines - I made sure I would have enough time to re-take the test and send out my new scores if need be before October 1st application deadlines. Good luck 🙂