Smell can really help - DH refilled the reed dispenser 2 weeks ago but put in the Christmas scent (very unexpected!!) - which filled the sitting room with lovely cinnamon and spice smells which was very evocative. (I had the bottle to put in sometime after today).
Lower lights - dimmer lamps with warm bulbs, candles, but lots of them - rather than harsh bright ceiling lamps - can also help.
Maybe put on a few Christmas songs - I've been listening to Country Christmas on TuneIn this week, which is a bit more gentle and relaxed music, rather than the "Christmas Classics Hits" that tend to be played on XmasFM or similar. So they're not too festive, but they are more relaxed and help my mind settle a bit.
And don't stress about it - whatever gets done, gets done. Whatever is not done, is not and that's fine too. Is there enough food that the people eating there will eat? Is the house warm enough, and clean? (Not sparkling clean - just comfortable).
But more importantly, are you overstressed (a la Brie Van Der Camp) about the season? Are you likely to take that out on others around you? What can you do to reduce the stress? It's far far better to have a lower key Christmas (in general, quite apart from this year in particular!) where people can relax and enjoy the quiet time, rather than be constantly rushing and the atmosphere being tense. This year in particular is a good one to say things need to change, and do things simpler.
Your tree is up - great.
You still have over 3 weeks to get anything that NEEDS to be done, organised and done.
The whole family can pitch in in different ways - smallies can tidy toys with you or stick on stamps to any letters/cards you are sending, older DCs can help by hoovering or you can teach them to peel the carrots or slice the onions, DH should definitely be doing his share (if you have a DH). Christmas is NOT a 1 woman operation!!
Presents do not need to be big and expensive. This is a good year to change the message to DCs on that, as they can see all the hard things happening around them this year and how lots of places can't do everything they normally would - and while Santa's workshop is still open and operating, they can't do as much as normal the same as everywhere else.
And maybe think about what you as a family would LIKE to do - is there a favourite walk nearby you can do together? A favourite movie you could watch some evening all together with some HM popcorn and a drink? A favourite board game you haven't played in a while?