When I called to make a small change to my reservation I asked this question because we wanted to do the same thing- MK early entry on Sunday morning. The CM I talked to did not know the answer so she found someone elase to talk to me and this lady told me we could check in at 7am. I asked if we could do it the Sat. night before since we were arriving in Orlando that evening as to save a little time in the morning on Sunday and she said NO it had to be the same date as the reservation to check in. HTH