When does fall start? The autumnal equinox explained

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The autumn equinox marks the end of summer and the beginning of fall in the Northern Hemisphere.

In 2025, the first day of fall will be Monday, Sept. 22.

During the equinox, the sun crosses directly over the equator, resulting in nearly equal hours of day and night.

The autumnal equinox marks the official goodbye to summer and the hello to fall. Scientifically, it signals the shift toward shorter days and longer nights as Earth's tilt leans us closer to winter.

In Arizona, however, fall feels a little different since triple-digit heat can linger well into September. So while the calendar says autumn, the desert sun takes its time letting go of summer.

Still, the equinox is a reminder that cooler mornings and crisper evenings are on their way.

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