It’s The First Day of Autumn
The September equinox occurs at 01:54 UTC. The Sun will shine directly on the equator and there will be nearly equal amounts of day and night throughout the world. This is also the first day of fall (autumnal equinox) in the Northern Hemisphere and the first day of spring (vernal equinox) in the Southern Hemisphere.
The September equinox marks the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from north to south and vice versa in March.