The first day of spring occurs on Saturday, March 20, this year. The vernal equinox, or the time when the sun is directly over a point on the Equator and the tilt of the Earth is negligible, occurs on Saturday at 1732 GMT, or 12:32 pm CDT. Contrary to popular belief, the equinox does not ensure an equal amount of daylight and darkness for every point on Earth. In fact, for Memphis, our days started becoming longer than our nights on Wednesday (March 17) when we had 12 hours and 1 minute of daylight (sunrise to sunset). On Saturday, the first day of spring, we'll actually get 12 hours and 8 minutes of daylight - pretty close to that "equal daylight and darkness" mark though, so you can understand the origin of the myth!
While Saturday will certainly feel like spring, with highs near 70, Sunday will be a much different story, as late winter returns for one last gasp. As typical with early spring though, as the sun moves to a position farther north of the Equator, it's rays tend to warm things up pretty quickly after a cold snap, and that is expected to occur early next week.