I am reading The Great Gatsby, and finding it difficult to grasp the meaning of "which comes at the two changes of the year" in the following sentences:
...One autumn night, five years before, they had been walking down the street when the leaves were falling, and they came to a place where there were no trees and the sidewalk was white with moonlight. They stopped here and turned toward each other. Now it was a cool night with that mysterious excitement in it which comes at the two changes of the year. The quiet lights in the houses were humming out into the darkness and there was a stir and bustle among the stars. Out of the corner of his eye Gatsby saw that the blocks of the sidewalks really formed a ladder and mounted to a secret place above the trees—he could climb to it, if he climbed alone, and once there he could suck on the pap of life, gulp down the incomparable milk of wonder.
Here, Gatsby is telling the narrator Nick how he and his lover Daisy walked down the street and kissed for the first time on an autumn night five years ago.
But I could not understand what were the two changes of the year in this context.
I would very much appreciate your help.