WeatherWhys for March 19,2014 from accuweather.com

Twice each year, the equinox occurs when the sun is directly overhead of the equator. This happens on the first day of spring and autumn. Spring this year begins on Thursday, March 20, at 12:57 p.m. EDT. The reason that the seasons do not exactly begin at the same time and date each year has nothing to do with the Earth's orbit, since the time it takes for the Earth to go around the sun is virtually the same from year to year. The problem is with the calendar, which does not entirely conform to nature.