December carries the two most widely observed bank holidays in the United Kingdom: Christmas Day on the 25th and Boxing Day on the 26th, both marked in all four nations. Where either falls on a weekend the day off moves to the following working day, so a Christmas at the weekend produces holidays running into the following week.
December closes the fourth calendar quarter and the calendar year itself on the 31st, resetting annual and quarterly reporting cycles at a stroke. The tax year runs on regardless, three months short of its own end, which is why December and April both feel like year ends to anyone doing the books.
The move from December into January takes the country over the turn of the year without a pause in the school holidays, which run on into the first week. December 2030 hands over the shortest days of the year to the month where the light starts coming back.