Autumn can be defined in two main ways: meteorologically and astronomically.

• Meteorological autumn always starts on September 1 and ends on November 30. This method divides the year into four equal three-month periods based on the Gregorian calendar.
• Astronomical autumn begins with the autumnal equinox, which typically falls on September 22 or 23, and ends at the winter solstice around December 21 or 22. In 2025, astronomical autumn starts on Monday, September 22, 2025, and ends on Sunday, December 21, 2025.

So, the exact timing of autumn depends on which calendar system is used, but the two main periods are September to November for meteorological autumn and late September to late December for astronomical autumn in the Northern Hemisphere.