Today is the Winter Solstice in the Northern Hemisphere. It is the day that a few things happen. The sun gets as far south as it gets. Today is the shortest day of the year. Tonight is the longest night of the year. Today is the first day of winter.
Since the winter solstice marks the start of the days getting longer, it was important to ancient people. There are various monuments that signal when the solstice is. These include Stonehenge, Newgrange, and even the Georgia Guidestones.