Anytime after 7:00 AM. You will actually be checking into your room, getting all your paperwork taken care of and given your room key and tickets. If you room is ready, you can drop everything off there. If it's not, you can call back later and get the room number. There will be no need to return to the front desk when you come back.
Leave your bags with Bell Assistance and go off to the park.
One note - make sure you get a resort map if you have to call back for your room number!!!! Then you can see where your room is located. You will have the general location but the map will narrow down your location further so you don't spend time wandering around finding your room. (Trust me on that one).