National Weather Service United States Department of Commerce

The winter solstice takes place at 10:28 am Thursday, December 21st. On this date, the amount of possible daylight is at its shortest amount. This does not actually line up with the earliest sunset and latest sunrise, though. The earliest sunset occurred at the beginning of the month, while sunrise will get a few minutes later before its latest point at the end of the month.

During the winter solstice, the Earth's axis is tilted at its farthest point from the sun, 23.5 degrees. While we're actually about as close to the Sun as we typically get each year, it is the low sun angle that causes this period to be colder than summer. 

Winter solstice information