The solstice is an actual astronomical moment, so who decides? SCIENCE.
Whether the solstices and equinoxes correspond with any sort of meteorological manifestations or cultural interpretations of the weather and nature is, yeah, a highly variable and subjective determination, so have at it. I would like to declare mini-seasons for the week when leaves continuously fall off the trees just after you thought you raked them all up; one that starts on the first day you hear the ice cream truck; and the week when you first have to start wearing a sweater and undershirt. There should also be movable ones, like the first binge-watch days of the year.
