I've done it both ways and prefer to prepare the ingredients ahead then put it together and bake it before serving. Now if you bake it tonight and reheat tomorrow, as long as you do it in the oven and not the microwave (or the top will not crisp), it will be good. But crisps are so much better when fresh from the oven!
Prepare the topping tonight and put in a covered container and refrigerate, peel the apples and toss with Fruit-Fresh or lemon juice (they will darken a bit, but after baking it will not be noticable)and refrigerate them as well. If you need to do anything else that may take a bit of time, like chopping nuts, do that ahead too. Then tomorrow night all you wil need to do it preheat the oven and add some spices to the apples, put it all together, and pop it in the oven.
I think I will now have to make one tonight... ;
Can't you just eat what I put in front of you? Do you have to know what it is?
Ria Parkinson, Butterflies (BBC, 1978-83)