The official start of fall, or the first day of autumn, in the Northern Hemisphere is marked by the autumnal equinox. This usually falls on September 22 or 23 but can sometimes occur as early as September 21 or as late as September 24. For 2025, the first day of fall is on Monday, September 22. On the equinox, day and night are approximately equal in length everywhere on Earth because the Sun is directly above the Equator. Meteorologists, for convenience, often consider September 1 as the start of the fall season, but astronomically, the equinox date is used to mark the official beginning of fall. So, in summary:
- Astronomical start of fall: Around September 22 or 23 (September 22 in 2025)
- Meteorological start of fall: September 1
This applies to the Northern Hemisphere. In the Southern Hemisphere, the seasons are opposite, so fall starts in March there.
