December 1st is the first day of meteorological or climatological winter (I guess depending on whether you are a meteorologist or a climatologist). You might wonder why it does not correspond with the astronomical season of winter, which is related to earth-sun geometry and starts this year on December 21. Here is a good article from Weather.com that describes the difference between the two kinds of seasons and why we climatologists think using the months of December through February is a better representation of the coldest time of year than the astronomical seasons does.
