The Winter Solstice occurs exactly when the Earth's axial tilt is farthest away from the sun at its maximum of 23° 26'. Though the Winter Solstice lasts only an instant in time, the term is also colloquially used as Midwinter or contrastingly the first day of winter to refer to the day on which it occurs. More evident to those in high latitudes, this occurs on the shortest day, and longest night, and the sun's daily maximum position in the sky is the lowest.
The seasonal significance of the Winter Solstice is in the reversal of the gradual lengthening of nights and shortening of days. Depending on the shift of the calendar, the Winter Solstice occurs on December 21 or 22 each year in the Northern Hemisphere
Worldwide, interpretation of the event has varied from culture to culture, but most cultures have held a recognition of rebirth, involving holidays, festivals, gatherings, rituals or other celebrations around that time. (From: Wikipedia, license: CCA-SA)