As of 2015, the United States will experience the December, or winter, solstice on Tuesday, Dec. 21, at 11:49 pm Eastern Standard Time. The December solstice usually occurs between Dec. 20 and Dec. 23.
The timing of winter and summer solstices depends on the Earth's position relative to the sun, and the location. The Earth is tilted along its north-south axis, and when it is winter in the Northern hemisphere, it is summer in the Southern. For example, Christmas in the southern country of Brazil is accompanied by tropical summer heat.
Solstices happen when the Earth's poles are tilted at their most extreme angles relative to the sun. During a winter solstice in the Northern hemisphere, the North Pole is angled 23.5 degrees away from the sun. This sharp angle makes for the shortest day of the year.
The United States is located in the Northern Hemisphere, so its winter solstice is in December. The date of this solstice shifts slightly from year to year, but it is always a few days before Christmas.