The shortest day of the year for the Northern Hemisphere is the Winter Solstice. On the solstice (usually between December 20-23)the sun is directly overhead the Tropic of Capricorn resulting in the North Pole being tilted the furthest away. This result in shorter days for sunlight in the Northern Hemisphere (and the further north you are the less sun you will see like those in Alaska or the Scandinavian countries). The reverse is true in the Southern Hemisphere. As the sun is closer to them, they celebrate the Summer Solstice. Those who live close to the South Pole will see Midnight Sun meaning they get 24 hours of sun during this time of year.
Many cultures observed the Winter Solstice because it marked an important time in the agricultural cycle. By this time all the crops and livestock had been prepared for winter. Important foodstuffs were stored for the months when virtually nothing grew. Wine and beer, which had been fermenting during the year, was ready at this time. Cattle and pigs would often be killed at the start of winter so they would not have to be fed during this time. The early months of winter were tough in many places and often called the “famine months” since little food was to be found. Many cultures observed the Winter Solstice as a renewal or that the year was reborn. For out of the seeming withdrawal of the sun, it would come back just as strong and powerful as before. Thus the Winter Solstice was seen by many as the start of a new year such as the old Roman feast of the Unconquered Sun (Sol Invictus) which happened around the 25th of December.
Today the solstice will be at 22 Dec 2015 @04:49 UTC, which is 23:48 (11:49 p.m.) in New York on 21 Dec 2015, 22:49 (10:49 p.m.) in Chicago, 21:49)9:49 p.m.) in Denver, 20:49 (8:49 p.m.) in San Francisco, 19:49 (7:49 p.m.)in Anchorage, and 18:49 (6:49 p.m.) in Hawaii.