Autumn Equinox 2025 Estimated Date. The first day of autumn often called fall (september equinox or autumnal equinox) is when the. Autumn equinox 2025 (beginning of astronomical autumn) on the day of the autumn (september) equinox, the sun, moving along the ecliptic, crosses the celestial equator, moving from the.

In 2025, the autumnal (fall) equinox arrives on monday, september 22, marking the official first day of fall. Just like the vernal equinox, this date and time are based on.