The single biggest mistake on the day before a big exam is to try and learn something new. Don't. Your brain is full.
Sleep. Eat the same meals you've been eating. Check the admit card. Pack the bag. Go for a walk. That's it.
On exam day: reach early, skip the crowd discussions, and trust your preparation. The paper will feel difficult — it's supposed to. Keep moving.
The first 15 minutes are for pacing. Don't panic on the first tough question; skip and return.
When it's over, it's over. Don't match answers in the corridor. Go home, rest, and focus on the next exam.