In the Northern Hemisphere, it’s the shortest day of the year
It’s known as the Winter Solstice.
The shortest day lasts for 7 hours 49 minutes and 41 seconds in London, UK, where I live. This day is 8 hours, 49 minutes shorter than the June Solstice. After this point, the days start getting longer and the nights shorter. Hurray!
This information is taken from a fascinating piece on the web site of the National Maritime Museum. You can find the full article here.