It’s the shortest day of the the year in the northern hemisphere!
OK, so today has the same number of hours, but there are now more hours of darkness compared to hours of light than any other day of the year.
From now on until the Summer Solstice (the longest day of the year), daylight hours will get longer and nights will come later.
If you’re within the North Polar region right now, you will be experiencing the Polar Night, where it is dark all day.