There are two positions of the Earth that occur on 21st March and 23rd September when the Equator receives the vertical rays of the Sun, and day and night are equal all over the world. These positions are known as equinoxes.
The vernal (spring) equinox occurs on 21st March and the autumnal equinox on 23rd September.
On 23rd September, it is autumn season in the Northern Hemisphere and spring season in the Southern Hemisphere. The opposite is the case on 21st March when it is spring in the Northern Hemisphere and autumn in the Southern Hemisphere.