Hiya, Philip!
Walt Disney World does currently require
theme park reservations for the
four main theme parks. I always recommend making those theme park reservations as soon as you can, because you don't want to be disappointed and not get into the park you want to go to on the day you want to go.
In looking at the
calendar for the days you'll be visiting, I can see that some theme parks are already booked full. This can change, Philip, as people do change plans and cancel reservations, so you can keep checking back to see if the days open up again. But to err on the side of caution, I'd start booking those theme park reservations now.
There are a couple of things to keep in mind, Philip. First, Orlando in August is really hot, so you can pretty much rely on any day to be a great
water park day. You don't need a reservation to go to the water parks, so you can choose any day that works with your schedule. Second, you may be able to change your theme park reservations if you need to. If you reserve your park days now, then see in the forecast that there's going to be a particularly hot day, you can check the calendar then and change your days around.
I guess this is kind of a long way to say yes, you can reserve your theme park day the day before, but you take a chance that the park you want will already be full for that day. It's better to be safe, reserve now, and move things around later.
Good luck, Philip!
Anna