An equinox, as defined by the National Weather Service, is all about the position of the sun in relation to Earth. As the sun passes that midline, the northern part of the Earth sees less of the sun, which causes shorter days and cooler temperatures, while the Southern Hemisphere enters spring with warmer weather and longer days. Most of the time, the fall equinox occurs on Sept. 22 or 23, but very rarely can occur on Sept. 21 or 24.