How many times a year does an equinox occur?

An equinox is an astronomical event in which the plane of Earth's equator passes through the center of the Sun, which occurs twice each year, around 20 March and 23 September.

The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the equator. As a result, the northern and southern hemispheres are equally illuminated.

In other words, the equinoxes are the only times when the subsolar point is on the equator, meaning that the Sun is exactly overhead at a point on the equatorial line. The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.