The seasons are concepts and
The seasons are concepts and require beings capable of conceptualizing to exist as such - ie "come to be called" what they are by humans. The natural occurrences that are referenced by the concepts were certainly there before they were conceptualized as Spring, Summer, Autumn and Winter. But to say they existed as such before they were conceived is certainly silly. Just get used to the fact that human beings rely on their senses (as, ultimately, do scientists) to determine what season they are in. And, yes, their onset varies. Astronomical events occur in seasons that had long been and still are defined without reference to them. The summer solstice is an astronomical event that occurs in Summer as commonly understood. Likewise the vernal equinox in Spring. "Vernal" means "in Spring". The seasons do not take their names or meanings from those astronomical events. The astronomical events take their names (though not their meaning) from the seasons.