Astronomically speaking, the seasons are defined by the Earth's tilt and orbit around the Sun. The four seasons are:
Meteorologists, on the other hand, define the seasons based on temperature patterns and climate trends. According to the meteorological definition:
No matter which definition you choose, December remains a unique "bridge" month—starting with the last crisp leaves of autumn and ending in the deep, quiet chill of mid-winter.
By December 1st, temperatures in much of the Northern Hemisphere have already dropped significantly. To a weather forecaster, waiting until December 21st to call it "winter" doesn't reflect the reality of the snow and ice already on the ground.