My Christmas dinner was on the table for about 1:30pm, and I didn't start cooking it until 9am (with most of the actual work not starting until more like 10:30). I did a little prep the night before, but not much really.
It's not really that much work when you've got it down. It's just a roast. If you can get it on the table for lunch time most Sundays, it's not that big a deal to do it on Christmas day.
This is the way.
Any big main meal is dinner, regardless of when you have it. A meal in the middle of the day is lunch, a meal at the end of the day is tea, a meal shortly before bed is supper. Any of them can be dinner too, but they don't have to be.
Breakfast is breakfast though. Dinner for breakfast is not an option.