I’m going to Walt Disney World tomorrow. Obviously, this means doing a lot of walking in the hot Florida sun. (Boo-hoo, you say.) I’ve been drinking a lot of water today in case I don’t get what I should tomorrow, but will it really make a difference, or will my kidneys have it all drained out by tomorrow morning? Is it worth it to drink a gallon or so of water today?
Too far ahead of time, will drain out. And may actually have the opposite of the desired effect. When you drink more water your body tends to retain less.
Drink extra water in the morning before you go out. But to stay healthy you really should try to drink water throughout the day. I have heard that Disney World has running water now. 
Drinking a lot of water now is a good way to ensure you wake up on time from bladder pressure.