"Equi" is "equal"; "nox" means "night". So, the equinoxes are days when the length of the daylight is equal to the length of the night. This happens twice a year, about March 21 and September 21 each year, when the Sun passes directly over the equator. (The precise dates may be a day earlier or later, depending on the cycle of leap years.) The U.S. Naval Observatory's "Earth's Seasons" web site lists the precise times of the equinoxes, solstices, perihelion and aphelion each year.