The Winter Solstice takes place on 21st December this year, with the date changing from year to year. Although the 21st also marks the Winter Solstice next year, in 2015 it will happen on the 22nd and then on the 21st again in 2016. The Winter Solstice is the shortest day of the entire year and marks the start of winter.
On the day of the Winter Solstice it sometimes appears as if the darkness that is normally associated with night-time, lasts not just all night but also all day. This is because the sun does not rise at the North Pole on this day; however, it is not the coldest day of the winter season, since the northern hemisphere will still continue to lose considerably more heat after this point than it will gain.
This carries on until very late in the winter season, when the start of longer hours of daylight starts to have a cumulative warming effect. For now, however, you should just throw another log onto the fire and get your hands and mouth around a nice cup of hot chocolate, as winter will still be carrying on for quite some time after the day of the Winter Solstice.
The ironic thing is that while the northern hemisphere experiences the shortest day of the year, the first day of summer will be being experienced by the occupants of the southern hemisphere as they bask in sunshine. The Winter Solstice has been a celebration of the new solar year for thousands of years.