The winter solstice this year is on December 21.  The winter solstice is the date on which the sun is lowest in the sky and the day is the shortest of the year.  It marks the start of astronomical winter (remember, climatological winter starts on December 1).  But it is not necessarily the day with the earliest sunset due to the orbit of the earth around the sun.  Here is a link to an article with some fun facts about the winter solstice from EarthSky.