Lovely decoration ideas here already.
In terms of buying ahead:
Definitely things like toilet roll, kitchen roll, paper napkins, etc. Are you planning on feeding everyone using your own crockery - do you have enough? Or will you use paper/plastic disposable crockery for a few meals and throw everything into a bin bag rather than loads of washing up?
Extra dishwasher tablets, and washup liquid. A good idea for maybe 2-3 weeks before Christmas is a bottle of dishwasher cleaner, so that it's spotless (and not smelling) - I'm always amazed at the difference.
Otherwise, think ahead now for all the different meals etc that you will have extras for, and numbers. Not just the (turkey?) dinner, but any breakfasts, lunches, other dinners, afternoon teas/cake and coffee visits, supper before bed etc. And while some of those might not be regular in your house, will some of your guests expect it (like a Horlicks before bed) or people calling in at odd times.
Nice to get now, while budgets are ok, is to put away a few nice bottles of wine for the main dinner, get a nice jam/marmalade for the relaxed breakfasts over the week, put away a few spare jars of a favourite sauce and tins/packets of favourite soup to make a couple of dinners in a hurry. (Like a sauce that you love to mix with leftover turkey, or quickly cooked chicken and serve with rice/pasta etc).
And general stores that you'll want extra of.
Tea, coffee, sugar, hot chocolate/cocoa/Horlicks etc
Dried herbs and spices, and stock (tins, cubes, concentrate - whatever you like to use), gravy powder
Biscuits - I tend to NOT buy the big tins, but enough packets of nice biscuits that I can open and serve a nice mix whenever I need to but don't have something calling to me (leftovers - come and eat me), and also can decide exactly what ones I'd like. And if we don't use them all, as I only open maybe 3-4 at a time, the rest stay fresh for another time.
Mixers for alcohol and drinks for non-drinkers - cola, lemonade, tonic etc, and also consider things like fruit squash for children, nice elderflower or grape juice or something else for adults, plenty of sparkling water too for various meals.
And also the alcohol that you may want - if you need to stock up on things, make a list and then you can pick up different things as they are on offer over the autumn.
Nice sharing packs of crisps - too big for ordinary use but perfect for tipping into a bowl when visitors arrive. Or breadsticks, those little salty sticks, Bombay mix, etc. (I tend to put out things like carrot sticks, olives and bowls of pesto and hummus as well but those are last minute buys - unless you make your own hummus so you can stock up on tins of chickpeas now!).
Any sauces - mustard (we like grainy mustard on Boxing Day ham sandwiches), horseradish for beef, mint sauce for lamb etc.
Part baked breads and rolls - for fresh bread for relaxed breakfasts!
Candles, batteries, matches, firelighters, those sorts of odds and sods. And plenty of bin liners for bathroom/bedroom bins (to change between any guests), and also for the main kitchen bin.
OK, a slightly chaotic meander through my brain of things I try to remember, but I hope it helps...