Meteorologists group seasons into months: March 1-May 31 for spring, June 1-August 31 for summer, September 1-November 30 for fall, and December 1-February 28 for winter. These groups make it simpler for meteorologists to describe the seasons. Not only are they easier to remember, they also correlate with temperatures for the seasons. The coldest months of the year in the northern hemisphere are typically December, January and February. The warmest months, on the other hand, are June, July and August. The graph below shows a comparison of the seasons, along with average temperatures during each season.
|source: NWS Kansas City|
So please, folks, quit saying that the equinoxes or solstices mark "the first day" of a given season. Because - unless you're an astronomer - they don't.