The first day of spring this year (2008) is March 20. The equinox will be at 5:48 GMT. That's 10:48 AM EDT if I figure correct. In general, the four seasons correspond to the relative position of the Sun to the earth. Meteorological determination of Spring is calculated when the Sun passes through the equatorial plane. When going from Winter to Spring, the Sun is moving North, and as soon as the Sun crosses the equator, we call it Spring.