Well, when I say ultimate .... 
But while the list isn't exhaustive, I hope you find it a good starter for ten.
Don't forget to post with details of anything I've missed!
Things to start doing, um, now, actually!
• Write gift lift and set your budget
• Write (and post) xmas cards
• Order xmas tree and make note of collection date. Did you know Hollickwood Primary School in Muswell Hill is selling ethically sourced, non-drop Nordman Fir trees at discounted prices (starting at just £25 for a 3-4ft tree) - local.mumsnet.com/Talk/local_barnet/2243980-Save-by-buying-your-Nordman-Fir-xmas-tree-from-Hollickwood-Primary-School
• Write a food and drink shopping list
• Pre-order shop-bought items eg. xmas cakes; puddings; anything else fancy!
• Buy non-perishables (
, nuts, gin, sweets, more .... !) so you can tick off your list and feel organised
• Secure a supermarket home delivery slot (they go quick!)
• Start gift shopping (and don't forget the gift wrap, tags and other bits and pieces)
• Wrap gifts as you go along (if you don't want to be up until 2am on Xmas day)
• If buying online, don't forget to shop using a cashback site (eg. I personally use kidstart)
• Post international xmas gifts and cards
W/c 1 December
• Put up your xmas tree (this is when you wished you'd pre-planned a colour/theme - so add that task further up list!)
• Book hair and beauty treatments so you look presentable for the big day. Why not visit our Offers page for a Mumsnet Barnet 10% discount at Kay and Kompany organic hair and beauty salon - local.mumsnet.com/barnet/offers?
W/c 8 December
• Put up xmas decorations - inside and out (but don’t worry if you couldn’t wait and have done this already!
• Clean the house (check out our listings section here if you can’t bear thinking about doing it yourself - local.mumsnet.com/barnet/cleaning-services)
W/c 15 December
• Produce a food preparation plan, complete with timings for cooking/preparing meals for the three days
W/c 22 December
• Buy fresh food (probably best to do the day before). Don’t forget the food for Santa and his reindeer 
• Buy batteries. Lots of them. (We’ve all been there!)
Christmas Eve
• Pour yourself a tipple of your choice, sit back and marvel at how organised you’ve been this year!