According to the astronomical definition, spring begins on the Vernal Equinox (usually March 20 in the Northern Hemisphere, and September 22 in the Southern Hemisphere), and lasts until the summer solstice (usually June 21 in the Northern Hemisphere and December 21 in the Southern Hemisphere). In 2009, Spring fell on March 20th in the Northern Hemisphere. According to this definition, therefore, the day called Midsummer’s Day in some traditions is the first day of Summer. Meteorologists generally define the beginning of spring as March 21 in the Northern Hemisphere and September 21 in the Southern Hemisphere. According to the Celtic tradition, which is based solely on daylight and the strength of the noon sun, spring begins in early February (near Imbolc or Candlemas) and continues until early May (Beltane). Unlike the other three seasons, people in relatively cool climates are likely to use the astronomical definition for the beginning of spring in popular jargon but retain the meteorological definition for the other three seasonal turning points.
The phenological definition of spring relates to indicators, the blossoming of a range of plant species, and the activities of animals, or the special smell of soil that has reached the temperature for micro flora to flourish. The first swallow to arrive for the flowering of lilac may be the indicator of spring. It therefore varies according to the climate and according to the specific weather of a particular year.
In Northern California, the arrival of spring is marked by the first day the weather is mild enough to bike to work in shorts. That would be today. So in our case, the astronomical and phenological are in perfect sync. ;-)