when is the first day of autumn

The first day of autumn in 2025 is Monday, September 22. This corresponds to the astronomical autumnal equinox, which occurs at 2:19 p.m. on that day. The equinox marks the time when day and night are approximately equal in length, signaling the start of the fall season in the Northern Hemisphere. Meteorologically, autumn also begins earlier on September 1, but the astronomical definition is most commonly used to indicate the official start of autumn.