Midnight Eastern Time.
Excursions do sell out - but a lot depends on what you're looking at and where you fall in the check-in pecking order. Some may never sell out, some (like Castaway Cay cabanas almost always do).
In terms of booking them "all" - you cannot over schedule excursions. There is a minimum amount of unscheduled time that must be met between scheduled activities. Personally if I'm planning to do excursions, I go ahead and book because you don't have to pay in advance and can always cancel them up to three days ahead of time.