Equinoxes and Solstices are formal seasonal approximations, as we have seen characteristic weather last well into another season and vice-versa.*
Just as natural wisdom indicates when the environment is dark, it is night; when light, it is day, no matter the numbers on the clock or calendar. While the full moon spans over a three night period )O( {Persephone, Lilith, Hecate; 666}.
However, such approximations are useful in the ceremonial & ritualistic observations of Satanic Sabbats {High Unholydays} & Esbats {full moons}