I was averaging higher on practice exams than how I actually performed on test day - this was also the case for almost ALL of my friends that have taken the MCAT too. However, my AAMC practice scores were much more consistent and closer to what I actually achieved.
Starting 6 days before test day, I took an official AAMC practice test each day, and reviewed the material I missed (or marked for review).
THE DAY BEFORE TEST DAY, I DID NOTHING.
I made it a point to take every single practice test prior to the big day, just to ensure I was scoring where I wanted to.
The prep course I used gave us practice tests from their own bank and access to old official AAMC MCAT tests. I found the official AAMC tests to be MUCH more similar to test day; I actually had a repeat question on test day that I had seen two days prior on a practice exam. You do not have to join a prep course to attain practice AAMC MCATs. You may purchase them at this website --> HERE
You're really not going to learn a lot more the day before the test - just rest, eat well, and get a good night's sleep. Just go to bed thinking of all the new knowledge you have acquired over the past few months - it will blow your mind.