Fall Equinox Day 2025 Usa. Autumnal equinox is the moment in which the sun enters libra. The four seasons of the year in the united states —spring, summer, fall, and winter — are what determine the weather, the ecosystem, and the hours of daylight throughout the year.

What is the beginning of the astronomical seasons: Equinoxes and solstices are two of the most important celestial events on earth.