Autumn begins on different dates depending on the definition used:
- Astronomical autumn begins with the autumnal equinox, usually around September 22 or 23 in the Northern Hemisphere. For 2025, autumn starts on Monday, September 22, 2025, and ends on December 21, 2025. This definition is based on Earth's tilt and orbit around the Sun and marks the transition from summer to winter with roughly equal day and night lengths at the equinox.
- Meteorological autumn begins on a fixed date for easier weather tracking and includes the months of September, October, and November. By this definition, autumn always starts on September 1 and ends on November 30 in the Northern Hemisphere.
Thus, autumn either begins on September 1 (meteorological) or around September 22–23 (astronomical), depending on the context. For 2025, the astronomical start is September 22.
