December 21st- The "Official" beginning of Winter
According to the Library of Congress: "Winter begins! The name "winter" comes from a Germanic term meaning "time of water" and refers to the seasonal precipitation. The winter solstice—the moment when the sun's apparent path is farthest south from the Equator—is used to officially mark winter's beginning. In the Northern Hemisphere, winter begins on the "shortest day" of the year, December 21 or 22, and lasts until March 20 or 22, the beginning of spring, marked by the vernal equinox, when day and night are equal in length"