Astronomical Spring (Vernal Equinox) began at 11:57 AM CDT today, March 20th. Due to the tilt of the Earth, this marks the moment when the most direct rays of the Sun cross the Earth's equator on its way north as the Earth orbits the sun. This marks the time that day and night are approximately the same length all over the earth.
Here's a more technical explanation from the U.S. Naval Observatory: Equinox