The winter solstice is considered the shortest day and the longest night of the year and this year it takes place on December 22 for the Northern Hemisphere.
Because Earth doesn’t orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly. The tilt of the Earth is what causes winter and summer. At the December solstice, the Northern Hemisphere is leaning most away from the sun for the year.