See, I know there are lots of people saying "it's only 1 day, a bit of a fancy roast and a few presents". But to me, Christmas is a season, not just a day. And it's also a season of goodwill and thinking of others as much as a day of gluttony and stuff. (And it's only a day of gluttony on years we travel "down home" and have to eat Christmas lunch with DMIL and Christmas dinner with DM!!)
I start out by looking at the entire season. It's busy at work and school and extracurriculars. The weather can be unpredictable. We will be travelling over the holidays, whether that's for Christmas Day itself or other days, so need to consider that. We have a very large family to buy for. A lot of family/friends overseas to send cards to. Various others that we support at the time.
And I also want to enjoy the season as well.
So I start out by looking at the diary and seeing what travel plans I need to facilitate, events for school/work/Cub scouts/other groups that must be fitted in, and what else is on locally that we want to go to.
It's busy, and I need to keep the house ticking over well. Clean clothes, clean kitchen to cook in, equipment needed for activities ready to go etc. So I do try and deep clean and declutter over the weeks beforehand (and get rid of toys DD no longer uses, clothes she's grown out of or that are needing replacement, old paperwork etc). Not all of that is needed because it is Christmas, but it is an opportune time to do it.
I buy things throughout the year if I see something that I know a particular person would love (particularly if it's in a sale - but just as often I just spot something), and more generic items on sales for teachers and other smaller gifts I need. I do have a spreadsheet that I have a page per year to keep track of my purchases and not give the same thing every year to people (apart from one who loves a book token!). And I have a box safely stashed for those purchases. I don't wrap until a couple of quiet nights in December, and just set myself up at the kitchen table with my tape dispenser, paper, ribbons, and some nice music to cheer me along.
We don't go overboard on extra food. We have some nice treats for ourselves (and the Lidl mini stolen bites, and larger stolen sliced with butter, are 2 of those!). I buy in extra packets of nice biscuits, tea, coffee etc so that we can be hospitable for visitors. I don't tend to buy the tins of biscuits as we would either not eat them and they go stale, or we would eat them unnecessarily (and I don't want to spend 3 months getting rid again in the gym). We also don't buy boxes of Roses etc as we are not great fans - but I will buy 1 box of Leonidas chocs, or a large bag of their orangettes, for all the family to enjoy spread out over the holidays.
If we stay at home, we go out visiting early in the day (mass, and various scattered relations near enough to us for tea etc), and then come home to cook turkey for just us (3). If we travel "home", we stay in a cottage near both sets of parents normally and travel for the day (mass, lunch at ILs, dinner at my DPs). Neither mother wants a dish for the dinner as they control their kitchens completely - but we will bring things to enjoy over the few days instead, like cheese and port.
I pick up bits and pieces that I will want over the weeks before Christmas - often not so much for Christmas day/dinner, but holiday items and general re-stocking of the cupboards, especially as there are often good deals on non-perishable items that I use anyway so it's a good time to have space in the cupboards for those. It's also a good time to buy extra for the local Lions' Club appeal - I always try to put together a "week's worth of food shopping for a family" so regular food (including breakfasts, dinners, snacks), bathroom and cleaning stuff, toothbrushes, nappies, with a few treat things added in rather than just a couple of tins and a box of roses.
I will have some extra wine/beer in, but it's not an alcohol fuelled festival. We usually have a party one day, and I make mulled wine for that and buy in party nibbles.
I am not the biggest prepper ever on making things and freezing them etc. DD and I make cookies anyway, so we always make sure we have made a batch in early December and freeze half the dough. Then, while we can make them from scratch on Christmas Eve if we have time (for Santa's snack), I can just slice and bake from the freezer batch if time is against us. I have breadcrumbs frozen for stuffing, and sometimes veg from the garden in summer or stock for gravy. But generally, if I am cooking myself, I peel potatoes and veg the night before and leave them in water, DH makes the stuffing on Christmas Eve as well, and then we do treat is it as a larger roast when we get home in the early afternoon.
We don't necessarily need a load of clothes "because it's Christmas". I usually wear something nice I have in the wardrobe, but I may have to add a couple of bits to replace older items to have a decent wardrobe for the whole period (work nights out, kids parties, family events and just casual wear away from work). DD may need a couple of bits as she's grown. DH looks after himself. The only thing I definitely get every year is new PJs for all 3 of us for a Christmas Eve box just before DD's bedtime - they are not necessarily a Christmas pattern but it is always good to have a nice new pair in the depths of winter.
My aim is to enjoy the season and not be stressed out. Most years, we manage a lot of that. So I usually go to work on Christmas Eve with a relatively light heart not panicked about the amount I still have to do at home or number of presents I still have to buy.