What does "which comes at the two changes of the year" mean here?

Question

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.

• F. Scott Fitsgerald, The Great Gatsby, Chapter 6

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.

Answer 1

TL;DR: Scott Fitzgerald is referring to Spring and Autumn.

The "two changes of the year" are the transition from dormancy to growth (corresponding to Spring), and from maturity to dormancy (corresponding to Autumn/Fall). Some countries define the start of these two seasons as the respective equinoxes. In a sense this is a somewhat arbitrary assignment, as the transition from one state to the other is a gradual one.

Nonetheless, it would be reasonable to say that by the equinox, the transition is well underway. In fact, amongst other things the equinox marks the point at which the rate of change (e.g. the lengthening of the day) is at its greatest.

By contrast, the summer or winter solstice marks the peak of growth or depth of dormancy, respectively. Far from being a time of change, they represent the maximum expression of the nature of that half of the year. The days are long (or short), and there's not much difference between a few weeks before the solstice and a few weeks after the solstice (other than an almost imperceptible change from lengthening to contracting, or vice versa). Those familiar with sine curves will recognise that the solstices represent the two points in the cycle at which the rate of change equals zero.

Answer 2

It's probably the solstice in December. One could consider the solstices as the two changes in the year (others may well consider the equinoxes to be the two changes). However, Christmas is around the December solstice, and Western story-telling often treats Christmas as a magical and exciting time, so I think this refers to the solstice.

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In Chapter 1, Daisy talks about the other solstice:

“In two weeks it’ll be the longest day in the year.” She looked at us all radiantly. “Do you always watch for the longest day of the year and then miss it? I always watch for the longest day in the year and then miss it.”

Prof. Barbara Hochman, in an essay on The Great Gatsby, feels that it is telling that the solstices are mentioned both at the beginning and the "climactic midpoint" of the novel. Her analysis of the book is interesting, you might want to read it.

• included in The Great Gatsby (Bloom's Modern Critical Interpretations), edited by Harold Bloom, 2010.