Which month marks the start of winter in the Northern Hemisphere?

Winter officially begins in the Northern Hemisphere on December 21 with the winter solstice – the day with the least amount of possible daylight and the longest night. For ease of recordkeeping, meteorologists and climatologists consider December 1 the first day of winter. But astronomically speaking, the Tropic of Capricorn (23.5 degrees south latitude) is aligned directly with the Sun on the winter solstice.

Source: Fox Weather