Winter officially begins when Earth experiences the shortest period of daylight and the longest night of the entire year. This phenomenon is called the winter solstice and it also marks the beginning of winter in the United States. For 2017, winter will officially start on December 21, 2017, Thursday, and will end on March 20, 2018 which will then be followed with spring. However, it is worthy to note that winter does not always begin on the 21st of December or not always on Thursday.
This holds true for the year 2015 when winter began on December 22 which was a Tuesday. To make it easy to understand, whichever date the winter solstice occurs, then that day will be considered as the official start of winter.