The first day of winter depends on the definition used:

• Astronomical winter in the Northern Hemisphere begins on the winter solstice, which is the shortest day of the year. For 2025, the first day of astronomical winter is Sunday, December 21, 2025.
• Meteorological winter, used by many meteorologists for easier recordkeeping, always starts on December 1 and lasts through the end of February.

So, the first day of winter can be either December 1 (meteorological) or December 21, 2025 (astronomical) depending on the context.