Today is the March/Spring Equinox. This equinox marks the moment where the Sun crosses the equator and usually occurs between March 19-21 every year. Both the March and September equinoxes are when the Sun shines directly on the equator making night and day nearly equal.
The March equinox is the transition from winter to spring in the Northern Hemisphere but the reverse in the Southern Hemisphere (summer into fall). Various cultures celebrate March equinox as a time of rebirth. Many spring festivals are timed to coincide with the equinox and some religious events (Passover and Easter) use specific calculations based on the equinox to help determine the exact day of the event.
Though the equinox marks the changing of the seasons, it is quite common for winter effects to continue in many places far until May or even June.