The seasons starting on the solstices and equinoxes are “astronomical” seasons”. The UK Meteorological Office, and I assume meteorologists and climatologists in general, for weather records, use “meterological” seasons starting of the 1st of the month containing the solstices or equinox – e.g. winter consists of December, January and February.
The beginning dates of these seasons as the 1st of the four months correspond to commenter Trebuchet’s suggestion, though I am still holding out for the 6th day.
The article says that professional meteorologists the world over use this system of ‘meteorological seasons’ but I had never heard of it before. It also says that Sweden and Finland use a season labeling system corresponding to temperature benchmarks.