I agree with atrain. Ideally if your preparation is timed perfectly you will not feel the need to look at things at all the day before, but that is not always the case. I would make a short list of the most crucial topics you still don't understand and look over them calmly and in a relaxed manner, trying to memorize the important concepts even if there are still some gaps in your understanding. I have never believed the "if you don't know it now you never will" theory, some of the things i looked at just before exams have shown up and i remembered them because they were fresh in my mind, but you certainly don't want to be stressing yourself out the day before the exam as that won't help at all, so if you do do some studying try to balance it out with another activity to give your brain a rest.