Although equinox literally means “equal night,” it isn’t exactly true that there are 12 hours of day and 12 hours of night everywhere on Earth during the equinox. Nor is the equinox defined by a specific calendar day; rather, it’s a specific moment in time that corresponds to planet Earth passing through a special point on its orbit.
If you were to draw an imaginary line through the Earth, from North Pole to South Pole, you’d find the line that represented Earth’s rotational axis. If you drew a second imaginary line, connecting the center of the Earth to the center of the Sun, you’d see that these two lines made an angle with one another. During the solstices, the Earth’s axis tilts maximally towards or away from the Sun. But during the moment of equinox, you make a right (90°) angle, which means that something very special occurs.