Yeah, I think your point about intensity of place is more salient than summer -- summer might signify only because many people seem to find it easiest to invoke this intoxication of the senses in recollection of childhood and of summer, together.

What I think /The Greengage Summer/ has is this quality of heightened sensory perception so intense that it becomes eerie, although nothing supernatural (or weird) (necessarily) occurs -- it's just the uncanniness of perception itself.

For me, quite mundanely, that time is the very end of summer and the very beginning of fall, when the light is changing, and just starting to darken at the edges, but there are still long evenings to be lived out by the fire.