OK, the calendars all say that the spring equinox was yesterday, but I always peg the equinoxes and solstices to the 21st of the appropriate month, mostly for convenience (but I have good company - the Gregorian calendar set the date at March 21, too). So as far as I am concerned, this is the first day of spring. The equinox is the time of "equal nights", when the night is the same length as the day, when the length of the day is changing fastest; for the next six months, our days will be longer than California's rather than shorter. Now we head into a month and a half of pure spring, before the early summer days start sneaking into the mix. Of course, around here that means that we have 6 weeks of mud season before it starts to dry out, not that the flowers are ready to bloom. But the snow is no longer accumulating, the light is changing, the days are getting longer, the catkins on the aspen are opening, birds are returning, the gophers are out, and spring is here.
It's interesting that summer and winter are primarily nouns (in spite of being used in sentences like "They summer in California" or "Where are you wintering your horses?"), while spring and fall are primarily verbs that have been applied to seasons; this goes all the way back to Old English, reflecting the reality of the year at the edges of the temperate zones. It is certainly true in Montana: winter and summer are relatively stable seasons with well-defined characteristics, but spring and fall are seasons of change between one extreme and the other, between long and short days, between ice and sun, between dormancy and explosive growth. In Chinese philosophy, spring is when the yin of winter is giving way to the yang of summer, and they are matched at the equinox; for one moment, life is perfectly balanced.