Spring occurs when the Sun’s rays strike the equator at noon from directly overhead. This particular time varies from year to year due to variations in the Earth’s orbit about the Sun. In the Northern Hemisphere the vernal (or spring) equinox (equi, “equal,” and nox, “night”) occurs between March 19 and 23, but usually on March 21.
During both equinoxes, all locations on Earth experience 12 hours of daylight and 12 hours of darkness, and the Sun rises due East and sets due West.