We don't cook the dinner on Christmas Eve, but we do most of the prep work then. Peel potatoes and veg, make the stuffing, (boil the neck for stock some years), brine the turkey if doing that, etc.
Our buffet meal is 24th - cured and cooked meats (parma ham, salami, braseola, etc), fish (squid rings, prawns, smoked salmon), olives, carrot and pepper sticks, cherry tomatoes, hummus (and usually another dippy thing), breadsticks and slices of crusty bread, cheese etc.
We have a decent breakfast on 25th, and then dinner in the evening. So we have mass and family visits to do in the morning and early afternoon, although we usually pop home before the last one to get the turkey into the oven (on a timer, just in case we get delayed).
Once we do get home, we throw a packet of M&S party food into the oven (something interesting that all 3 of us will enjoy), get the rest organised and light the fire, before everyone gets a drink and we head into the sitting room with the M&S bits. We open presents at a relaxed pace, topping up drinks, and making an occasional foray into the kitchen to check the dinner or turn on the veg or whatever the next stage is. The M&S food is effectively our starter, we are generally hungry enough by the time we eat to enjoy the dinner, and then DH and I usually have proper pudding later once DD is in bed (she doesn't like the fruit) and we have digested the turkey a bit.
For us, it's about slow eating of things we enjoy, (and having lots of nice things but not going overboard on the amounts of anything), and having a slow day - which is very rare in our house!