Winter doesn’t officially begin in the Northern Hemisphere until around December 21st. Known as the Winter Solstice, this is when for locations north of the equator the Sun is the lowest in the southern sky making it the shortest day of the year.
Skywatchers will notice that throughout the fall season, the Sun’s midday position in the sky slowly sinks closer to the horizon, making for ever-longer shadows. Meanwhile, the Sun appears to move toward the south day by day, rising farther from the east and setting farther from the west as we approach the winter solstice. During winter the Earth's northern axis is slightly tilted away from the Sun and so the Northern Hemisphere receives less sunlight.
On the first day of winter the Sun rises as far south of due east as it gets for the year and for the next few days the Sun appears to rise at the same place on the horizon. This moment in the year is when the shadows stretch their longest, and we get the fewest daylight hours. After the solstice the Sun’s pathway across the sky appears to head back in the opposite direction until it reaches its northern limit during the summer solstice in June.
The exact date and time of the winter solstice, while always occurring within a day or two of December 21, changes from year to year because of the difference between a calendar year of 365 days, and the solar year of 365.26 days - the exact time it takes for the Earth to make one trip around the Sun.
Source: Weather Network.com