21st December 2018 is the darkest ( or shortest) day of the year. The day celebrates the winter solstice in the Northern Hempisphere and the Summer Solstice in the Southern Hemisphere.
The sun is situated directly over the Tropic of Capricorn in the Southern Hemisphere- its most southern declination of -23.4 degrees. This essentially means that the sun is at the furthest point away from the North pole, tilted away from the sun. Therefore the sun is above the horizon in the northern hemisphere for the shortest period of time and this day is considered to be the 1st day of winter by astronomers.
So get prepared to deal with the darkness, plan a duvet day, get the heating on or if you are lucky enough get away to a sunnier climate! Unless of course you love the winter and darkness – in which case embrace it!