This year's autumnal or September equinox occurs at 03:09 Universal Time (UTC) on September 23. In my local time, Eastern Daylight Time (EDT), it's 23:09, 11:09 PM, on September 22.
The Earth's seasons are caused by the rotation axis of the Earth not being perpendicular to its orbital plane. The Earth's axis is tilted at an angle of approximately 23.44° from the orbital plane; this tilt is called the axial tilt. As a consequence, for half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum around June 21, while for the other half of the year, the southern hemisphere has this honor, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes.
- Wikipedia: Equinox