The winter solstice occurs some time between December 20 and December 23 each year in the northern hemisphere, during the longest night of the year. This year the Solstice was on the 21st at 12.04. Next year it will be at 5.47 pm. From this moment the days will gradually lengthen!! But it also marks the beginning of a our winter.
The winter solstice occurs at the instant when the sun's position in the sky is at its greatest angular distance on the other side of the equatorial plane from the observers hemisphere.
Worldwide, interpretation of this moment has varied from culture to culture, but most cultures have held a recognition of rebirth, involving holidays, gatherings, or rituals. In Europe the solar calendar midwinter celebrations still centre upon the night of December 24 leading into the December 25, which was considered to be the winter solstice upon the establishment of the Christian calendar Julian. Some facts: The Julian calendar, a reform of the Roman calendar, was introduced by Julius Caesar in 46 BC. The Julian calendar remained in use until Pope Gregory XIII decided to opt for the Gregorian calendar (around 1582). The Gregorian calendar is the most widely used calendar in the world today. It is an arithmetical calendar. But basically it is a reform of the Julian calendar counting the years from the incarnation of Jesus.
This is why we celebrate Christmas on the 25th.