Autumn starts either on September 1st by the meteorological definition or around September 22nd or 23rd by the astronomical definition, depending on the context.

Meteorological autumn

• Defined by meteorologists for consistency.
• Runs from September 1st to November 30th every year.
• Divides the year into four equal three-month periods: spring, summer, autumn, and winter.

Astronomical autumn

• Determined by the Earth's axis tilt and orbit around the Sun.
• Begins with the autumnal equinox, around September 22nd or 23rd.
• Ends with the winter solstice around December 21st or 22nd.
• In 2025, astronomical autumn begins on Monday, September 22nd, and ends on Sunday, December 21st.

Thus, autumn can be described as starting on either September 1st (meteorological) or around September 22-23 (astronomical).