1st dinner seating = 1st breakfast on debarkation day, about 6:45 am
2nd dinner seating = 2nd breakfast on debarkation day about 8 am
in the same restaurant you had dinner in on your last night. If you ate in Palo, Cabanas, or Remy on your last night then it is the MDR you would have eaten in on your last night.
If you have 2nd dinning and need the early breakfast, Cabanas is an option or talk to your server to see if you can switch to the early breakfast. We had no problem switching to the early breakfast but we weren't on a full ship either.