You may have noticed the countdown to spring in Chicago in the right hand column and seen the date as March 19th. You may be saying wait the first day of Spring always arrives on the 21st of March. Well that is not necessarily true. Traditionally the beginning of spring has been celebrated March 21 but now things are changing. Calendar makers and astronomers now say that the spring season starts one day earlier, in all time zones in North America. That would mean we would celebrate it on March 2oth.
The vernal equinox or spring equinox occurs when the sun rises exactly in the east travels through the sky for 12 hours and sets exactly in the west. Our calendars are not exactly attuned to the tropical year which is the actual length of time it takes for the earth to complete one orbit around the sun. So strictly speaking the first day of spring depends on your location. In the years 2008 and 2012, if you live in Alaska, Hawaii and the Pacific, Mountain and Central time zones you will see will see spring begin on March 19. And in 2016, it will start on March 19 for the entire United States.
If you would like a bit more information please check out this article Why Has Date Changed for Start of Spring?