I’ve often thought there were three ways to define the seasons. I’ll use spring as an example. The first are the astronomical seasons, which is what calendars show and in my example would be the date of the spring equinox. The second is the meteorological seasons, which where I live says spring are the months of March, April, and May.
I’ve always said there was a third way to define spring (and other seasons), it’s spring when it starts to look and feel like spring. For example I like to watch for the first crocus or tree buds. While I’m not being precise, is this in general what would be meant by a phenological season?