The first day of spring (Spring Equinox) is today; March 20th at 6:21 pm CDT (2321Z).
There are two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in an equal amount of daylight and darkness at all latitudes. These events are referred to as equinoxes and occur near March 21 (Vernal Equinox) and near September 21 (Autumnal Equinox). At the equator, the sun is directly overhead at noon on the two equinoxes. The Vernal Equinox is at 6:21 pm CDT (2321Z). The Autumnal Equinox will occur at 4:05 am CDT (0905Z) on September 23, 2011.