Dear Sunken,
Have you ever questioned the validity of “the first day of summer”, when it is at the top of ‘the roller coaster ride’?
Now, have you questioned the validity of ‘the first day of Winter’, when it is at the exact bottom of the roller coaster ride?
Now, to finish, consider the equinoxes, the platforms that you enter and exit the roller coaster cars.
graph:
Yeah, actually that has bugged me since I first found out how the British refer to it as “midsummer’s day” — the Earth hasn’t warmed up enough around here to do planting until a way into May, and there’s a concern about a hard frost until after May 1, which (with “midsummer” being in the middle) would fall after the beginning of summer.
In the US, the summer begins with Memorial Day weekend, and ends with Labor Day weekend.