Today (December 21st) is the Winter Solstice, which is officially the “shortest” day of the year!
While today marks the least amount of daylight we see for the year here in the Northern Hemisphere, it also means that the nighttime is the longest of the year. Close to the North Pole, there is darkness 24 hours a day! Here is a picture that shows how that works: https://goo.gl/images/1paZiy
The opposite of the Winter Solstice is the Summer Solstice, which will be on June 21st. When it is time for the Summer Solstice at the North Pole, there is daylight for 24 hours a day!
If you want to learn more about how the Winter and Summer Solstices work, you can check out this YouTube video: