In astronomy, the Winter Solstice in the northern hemisphere is the date when the sun reaches its southernmost point in the celestial sphere. It is the date with the shortest day and hence with the longest night. This date usually falls on December 2/December 22 (in the northern hemisphere) or June 2/June 22 (in the southern hemisphere).
At the time of this solstice, the earth is in that point of its orbit at which the hemisphere in question is most tilted away from the sun, causing the sun to appear at its farthest below the celestial equator when viewed from earth.
In some reckonings, the Winter Solstice is the first day of Winter. In Chinese calendar, for example, the Winter Solstice is called dong zhi(winter's arrival) and is regarded in certain Chinese areas as an equally, if not more, important Jie Chia compared to the Chinese New Year.