I'd start with making sure you've got plenty of the boring, necessary stuff like loo roll, bin liners, dishwasher tabs, washing up liquid, batteries etc. It might be worth a trip to somewhere like Costco to stock up on these.
If you have a garage, consider if it's worth installing an extra freezer. We got one for our dogs (raw fed) but it's brilliant at Christmas as it gives a lot of extra space and I can keep things in there that I've prepped or bought in advance. Some puddings, home made soup, things like that.
Draw up a rough menu plan for the time you have guests. For each day, I'd work on breakfast, a light lunch and a main meal. Don't assume you have to cater every single meal - for that length of time I'd consider a takeaway one night (that could be really fun, if you have a pizza party) or eat out one night. That means you only have four main meals to prep/cook, and I'd also look at lunch out at least once or twice too. The breakfast and lunches can be light and straightforward and don't need to involve too much effort from you. So breakfast Day 1 - croissants. Day 2 - home made granola Day 3 - porridge (can be made in slow cooker overnight) Day 4 English muffins and scrambled eggs Day 5 Pain au chocolat or Danish etc.. Serve with plenty of fruit juice, tea, coffee, fresh fruit. I'd have available a couple of cereals. Most of these can be either prepped ahead or frozen and defrosted the night before. Again, Costco (or similar!) is your friend.
I'd also consider activities over the week - maybe think about booking a panto trip, any Christmas theatre/ballet/concerts, skating, Santa train rides, guided walks etc - this will get you all out of the house. Fresh air and a change of scene very welcome.
Agree with pp about reviewing your crockery, cutlery, glasses, towels and bedding. Work out where everyone is sleeping, and what they are sleeping on. Have you got enough pillows/sheets etc? Enough chairs?
Ikea is brilliant for cheap basic china, glasses etc and a couple of extra spare folding chairs.
Plan and book in a deep clean for the house before the guests arrive (may be worth getting this booked via an agency) and windows/oven cleaned. Also consider the space issue of decorations, cards etc when teh house is full. May be worth trying to keep these out of the way/minimal to allow plenty of space/circulation room.
Once you've worked out your menu, tell people what you'd like them to bring so you can cross that off your list. e.g. Uncle Bert can bring savoury crackers and a really good Cheddar, cousin Mabel is bringing Christmas cake and her famous home made sausage rolls etc.
Alcohol can be an expensive part of the hosting so find out if you don't already know the preferences of your guests, so that you only have in drinks people like and will consume. Nothing worse than being left with a random selection of dodgy sherry and prosecco if you don't like it. Broad rule of thumb is half a bottle of wine per adult for evening meals, so a meal for six would probably involve three bottles but you should know your guests. It's often easier to buy by the case if you are hosting. Considerate guests will ask what you would like them to bring - don't be afraid to have lists or ask for specifics.
Good luck!