In astronomy, the winter solstice is the moment when the earth is in that point of its orbit at which one of the earth's hemispheres is most tilted away from the sun. This causes the sun to appear at its farthest below the celestial equator when viewed from earth. Solstice means "The Sun stands still" , i.e., its noontime elevation does not seem to change from day to day.
The date of the winter solstice is the date with the shortest day and the longest night of the year. In the northern hemisphere, this date usually falls on December 21/December 22, while it falls June 21/June 22 in the southern hemisphere. By definition, these dates are also the dates of the summer solstice in the opposing hemisphere.
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. The winter solstice is also the time when the Celtic festival of Yule was celebrated, and is still celebrated today as a Neopagan Sabbat.
In addition to the above, many cultures celebrate or celebrated a holiday near (by pratically a few days) the winter solstice, examples of these include Saturnalia, Christmas, Hannukah, and Kwanzaa.
See also: List of winter festivals, season, sun mythology