It happens every year between December 20th and the 23rd, the first day of winter - the Winter Solstice - and this year it falls on Saturday, December 21, 2013 at 17:11 UTC.
This is a time when the sun reaches its most southerly declination of -23.5 degrees. In other words, it is when the North Pole is tilted 23.5 degrees away from the sun.
When this happens, every place on earth above a latitude of 66.5 degrees north remain in darkness, and below this latitude, get 24 hours of daylight. The winter solstice marks the shortest day and the longest night of the year.
You can use the Sunrise and Sunset calculator to find the number of daylight hours during the December solstice worldwide.
Winter solstice happens when the sun is directly over the Tropic of Capricorn in the southern hemisphere.
It is a time of celebration for many cultures because it represents the end of winter's darkness and the coming of spring. For many ancient civilizations that struggled to survive through harsh winter, the winter solstice was a time to be grateful for winter passing.
Festivities and rituals, all over the world celebrate this significant change.
In the NE corner of Pakistan and among the Kalash Kafir people, a seven day celebration takes place that includes the Winter Solstice. They take ritual baths for purification, they sing and chant with a torchlight procession and there is dancing and festival foods.
The Mayan honored the sun god prior to the Christian influence, doing a dangerous ritual known as the flying pole dance.
The ancient Incas celebrated with a special festival to honor the sun god.
In the 16th century these kinds of ceremonies were banned by the Roman Catholics in their attempt to convert the Inca people to Christianity. However, the Quecia Indians in Cusco, Peru, brought back the festivals and celebrations in the 1950s. It is now a major festival that begins in Cusco and continues to an ancient amphitheater nearby.
In the Neolithic and Bronze Age, Winter Solstice was a way for people to identify the actual time for harvests and sowing of new crops.
However, the most common modern-day tradition of winter or summer solstice observed around the world is to view sunrise and sunset.
Image via NASA