If I prepare this the day before, how would you suggest reheating and serving?

The French are known to eat ratatouille cold, at room temperature and warm.

I would just put in a saucepan and reheat over low heat. You can also use a microwave oven if that suits your fancy and/or you are in a rush.

Ratatouille is basically vegetable stew.

Put in a serving dish and let guests serve themselves or plate it with the other items on the dinner plate. Up to you.