What is a equinox

What's the context ?

The equinox is when the plane of the earth's equator is in line with the sun. The result of this geometry is that the length of night = the length of day for all points on the earth. It typically occurs on or around March 21 and September 22, and coincides with the astronmical dates for the first day of spring and fall. For a more detailed explanation see:
Equinox - Wikipedia, the free encyclopedia