Today is the Vernal Equinox. Here's some information about that:
Far from being an arbitrary indicator of the changing seasons, March 20 (March 21 in some years) is significant for astronomical reasons. On March 20, 2008, at precisely 1:48 A.M. EDT (March 20, 05:48 Universal Time), the Sun [crossed] 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 2008, it will be on September 22 at 11:44 A.M. EDT (15:44 UT). This date will mark the autumnal equinox in the Northern Hemisphere and the vernal equinox in the Southern (vernal denotes "spring").
Modern astronomy aside, people have recognized the vernal equinox for thousands of years. There is no shortage of rituals and traditions surrounding the coming of spring. Many early peoples celebrated for the basic reason that their food supplies would soon be restored. The date is significant in Christianity because Easter always falls on the first Sunday after the first full moon after the vernal equinox. It is also probably no coincidence that early Egyptians built the Great Sphinx so that it points directly toward the rising Sun on the day of the vernal equinox.