Always been dinner to me.
Coming from a working-class Lancashire family as I do, dinner was always the midday meal and we had tea, which may have been a simple cooked meal like egg and chips, when dad got home from work or at about 6pm at weekends. We had Christmas Dinner round about 2pm (when the sprouts were boiled to death).
Later, after I'd been to university, left home, got married and realised that you don't have to do what your parents did, it was lunch at midday and dinner in the evening. Even later, after I'd married for the second time of asking and was living in central(ish) London, it was lunch midday (brown bag or something from the sandwich shop) and supper in the evening if eating at home, and dinner was when we went out to eat. That's still pretty much my model, it's a city-slicker's way I admit.
But Christmas Dinner has always been Christmas Dinner. It's a special meal, like one eaten out, and a one-off in the year.