Autumn starts on different dates depending on the definition used:

• Meteorological autumn begins on September 1 and ends on November 30. This is based on splitting the year into three-month periods aligned with the calendar for consistent weather observations and statistics.
• Astronomical autumn begins with the autumnal equinox, when the sun shines directly on the equator and day and night are approximately equal in length. For 2025, the astronomical autumn starts on Monday, September 22, and ends on Sunday, December 21.

Both definitions are widely used, with meteorological autumn starting clearly on September 1 and astronomical autumn defined by the equinox around September 22 or 23.