Today is the first day of Spring, the on which the Vernal Equinox falls.  So what exactly is the vernal equinox?
This is one of the four days each year that define the change of seasons.  Spring and autumn begin on equinoxes while summer and winter begin on solstices.

The Vernal Equinox is as astronomical phenomenon when the tilt of the earth's axis is exactly the same at the sun's.  So our equator and the equator of the sun appear on the same plane.  This year that occurred at exactly 5:20 GMT.   The equinox is also the day where we have an equal amount of sunsine and darkness - about 12 hours of each.