The Beginning of Spring vs. the 1st Day of Spring

Today is the beginning of spring. It began at 1:26 pm EST. Tomorrow is the 1st full day of spring. The vernal equinox is when the sun crosses directly over the Earth's equator. This moment is known as the vernal equinox in the Northern Hemisphere. For the Southern Hemisphere, this is the moment of the autumnal equinox.
Translated literally, equinox means "equal night." Because the sun is positioned above the equator, day and night are about equal in length all over the world during the equinoxes. A second equinox occurs each year on September 22 or 23; in 2006, it will be on September 22 at 11:03 P.M. EDT (Sept. 23, 03:03 UT*). This date will mark the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern Hemisphere.