One of two days during the year when the declination of the Sun is at the equator. The vernal equinox denotes the first day of the spring season. For the Northern Hemisphere, the date of vernal equinox on either March 20 or 21 (changes yearly). September 22 or 23 is the date of the vernal equinox in the Southern Hemisphere. During the vernal equinox, all locations on the Earth (except the poles) experience equal (12 hour) day and night.