It's Winter Solstice.

Today, the Northern Hemisphere will be tilted as far away from the sun as it will be all year. It’ the first day of winter, it’s also the shortest day — and longest night — of the year. This year’s winter solstice occurs at exactly 11:48 p.m., Eastern Time.  In astronomy, the solstice occurs at the moment the earth's tilt away from the sun is at a maximum. On the day of the winter solstice, the sun appears at its lowest elevation with a noontime position that changes very little for several days before and after the winter solstice.