We must slow down and look over our shoulder. To take a look at the year's achievements. What is worth something and what we should get rid of.
We must think about our family and friends. Must send good thoughts.
We must go to every possible Christmas markets and have hot drinks.
We must cook delicious meals traditional to the end of the year so as to make it definitely clear that it is Christmas or New Year's. I enjoy the Finnish laatikko - oven-baked mashed root vegetables with various spices. A genuine Finnish Christmas dish, the main one. The Finns eat some ham too (sometimes about 10 kg of it) and special vegetable salad only on Christmas, and afterwards, until it is finished (which can drag way into the next year).
To create an ambiance, you can light some candles, if you're lacking other props.
You must get at least one calendar for the next year. I've got one!
And must drink wine. Mulled and same as usual.