Occurs around September 21st–24th. The Sun crosses the celestial equator moving southward. Again, day and night are roughly equal globally. This begins astronomical autumn in the north and spring in the south.
The four seasonal "starting points" are not arbitrary dates. They are astronomical events, defined by the Sun’s apparent position relative to the celestial equator (an imaginary projection of Earth’s equator into space). These events are the two and the two equinoxes . when are the seasons