Beyond science, the start of winter is often defined by what people see and feel:

When does winter start and what is the solstice? - BBC Weather

For the purists and the astronomers, winter begins on the Winter Solstice, usually falling on December 21st or 22nd in the Northern Hemisphere.

Winter is more than just a drop in temperature; it is a celestial and atmospheric event with multiple "start" dates depending on whether you are looking at the stars or your local weather station.