Today, Friday, March 20th, is the official first day of spring for 2009.
Spring is always between March 19th and March 21st, based on the Vernal Equinox — when the sun is directly above the equator. Since the sun rises from the east and sets due west and our 365 day calendar isn’t perfectly precise, the date changes.
Today the sun will cross to the northern hemisphere at 11:47 pm-ish. Spring will be official at that moment.