Sunday is Early Magic Hour at MK. If you are staying on-site, it would be a good idea to go right when EMH opens, especially if you are going close to Christmas. You will probably want to switch parks mid-day as it gets very crowded on the EMH days.
If you're not staying on site or don't want to rise that early, Monday would be fine. I would also recommend getting there at opening. We went on the Monday before July 4th, and the crowds were light in the morning but got heavier as the day went on.