Which phenomenon occurs when the sun crosses the plane of earth’s equator?

The sun appears to cross the plane of Earth's equator when it reaches either one of the "equinox" points in the sky.  In ancient times, the lengths of day and night are equal everywhere (except at the poles), and the occasions are marked as the beginning of Spring and Autumn. 
ANSWER
THE EQUINOXES IS THE PHENOMENON THAT OCCURS WHEN THE SUN CROSSES THE PLANE OF EARTH"S EQUATOR - Autumnal and Vernal (or Spring). The hours of daylight equal the hours of darkness