Today will be the shortest of the year - the winter solstice.
The solstice marks the moment the sun shines at its most southern point and has been celebrated by pagans for thousands of years.
Many of the traditions now associated with Christmas had their roots in winter solstice celebrations - including the Christmas tree.
What is it?
The winter solstice marks the shortest day of the year and the official beginning of winter. The solstice itself is the moment the sun is shining farthest to the south, directly over the Tropic of Capricorn.
When is it?
In the UK, the precise time of the solstice will be 5.11pm on Saturday December 21st 2013.
The time is different every year, falling between December 20th and 23rd.
The winter solstice is a major pagan festival, with rituals of rebirth having been celebrated for thousands of years.
Every year revellers gather at Stonehenge to watch the sunrise on the shortest day.
Many of the traditions we now think of as being part of Christmas -including Yule logs, mistletoe and Christmas trees - have their roots in the pagan celebrations of winter solstice.
Wait, the Christmas tree was originally a winter solstice tree?
Sort of. The Druids - the priests of the ancient Celts - used evergreen trees , holly and mistletoe as symbols of everlasting life during winter solstice rituals. Cutting them down and putting them in their homes would have been too destructive to nature.
The idea of decorating trees in the festive season was a combination of the Druidic rituals and a German tradition, which according to legend was started by Martin Luther.
Today will see the fewest hours of sunlight of the year - just 7 hours and 49 minutes. The days will gradually get longer from here until the summer solstice on June 21st 2014.
So, is the world going to end?
Probably not. Last year, thousands of people showed up at Stonehenge for the winter solstice celebrations because it was thought that the date marked the last day of the Mayan calendar, signalling the end of the world.