While snow marks the beginning of winter for many people, the first official day of winter is Sunday, Dec. 21, known as the December solstice.

It's a point in time that marks a transition in our planet's annual trip around the sun.

The sun comes up each day because Earth rotates once on its axis every 24 hours or so. Seasons, and the arrival of the solstice, are a result of Earth being tilted 23.5 degrees on its spin axis coupled with the planet's 365-day orbit around the sun.

More: http://www.livescience.com/environment/081219-winter-solstice.html