Because I am a geek for all things to do with astronomy and ancient customs...
While it’s true that we’ve traditionally celebrated the beginning of spring on March 21, astronomers and calendar manufacturers alike now say that the spring season starts one day earlier, March 20, in all time zones in North America. Unheard of? Not if you look at the statistics. In fact, did you know that during the 20th Century, March 21 was actually the exception rather than the rule?
The vernal equinox landed on March 21, only 36 out of 100 years. And from 1981 to 2102, Americans will celebrate the first day of spring no later than March 20.
In the years 2008 and 2012, those living in Alaska, Hawaii and the Pacific, Mountain and Central time zones will see spring begin even earlier: on March 19. And in 2016, it will start on March 19 for the entire United States.
There are a few reasons why seasonal dates can vary from year to year.
A year is not an even number of days and neither are the seasons. To try and achieve a value as close as possible to the exact length of the year, our Gregorian Calendar was constructed to give a close approximation to the tropical year which is the actual length of time it takes for the Earth to complete one orbit around the Sun. It eliminates leap days in century years not evenly divisible by 400, such 1700, 1800, and 2100, and millennium years that are divisible by 4,000, such as 8000 and 12000.
Another reason is that the Earth’s elliptical orbit is changing its orientation relative to the Sun (it skews), which causes the Earth’s axis to constantly point in a different direction, called precession. Since the seasons are defined as beginning at strict 90-degree intervals, these positional changes affect the time Earth reaches each 90-degree location in its orbit around the Sun.
The pull of gravity from the other planets also affects the location of the Earth in its orbit.
The current seasonal lengths for the Northern Hemisphere are: