Why does winter begin in December?

Answer 1

The winter solstice is the minimum in heating, but the minimum in temperature lags because of thermal capacitance of the atmosphere-ocean-land system.

Climatologists usually consider (northern) winter to be December-January-February, which is three weeks ahead of the astronomical definition. That puts the middle of winter in the second week of January, which is pretty close to the time of the historically observed temperature minimum, at least in locations far from an ocean.

Answer 2

winter begins in December because that is when the solstice occurs when countries north of the Equator are tilted the furthest away from the sun. It is also the beginning of summer in countries south of the Equator

Answer 3

The Roman Christians did it. It was decided, much after the death of Jesus Christ, that he was born on the winter solstice - though there is no evidence. This fits nicely with numerous other pagan religions of the time, which celebrated extravagantly the beginning of the lengthening daylight in the northern hemisphere. In that time, the winter solstice was on December 25. The solstice moved around, having a different date every few decades, because the calendar being used then was not exactly 365.25 days. When the Gregorian calendar was established, the New Year (January 1) was decided to be eight days after December 24/25, reflecting the belief that Jesus was circumcised on the eighth day after his birth, as was the Jewish custom. This would be the first time that Jesus would shed blood, demonstrating his humanity and initiating his path of spiritual salvation for humankind.

My preference would have been to have the new year begin on the winter solstice (I reside in the Northern Hemisphere), but myth and magic ruled the day.