The Autumnal Equinox
This year, the autumnal equinox falls on the 22nd of September. It typically occurs on the 22nd or 23rd but, due to differences between the calendar year and the solar year (365 days versus 365 and 1/4 days), may take place anytime between Sept. 21st and Sept. 24th. The last time an autumnal equinox was on the 21st however, was in 1931. And the next Sept. 21st equinox isn’t until 2076. The last time one occurred on the 24th was in 1907. That won’t happen again until 2303.
An equinox takes place when the Earth’s axis is turned neither away from nor towards the sun, which when seen from the equator rises due east and sets due west. Day and night are of approximately equal length everywhere in the world. The word equinox comes from the Medieval Latin word equinoxium, which means equality of day and night.