John Marcher's name suggests March, the beginning of spring, a time of newness and rebirth. He truly believes something, either great or terrible, will happen to him. In any case, this "something" will be new and is thus related to the idea of spring. At first, May suggests this "something" will be falling in love. John dismisses this. John thinks he is involuntarily "marching" toward his own fate, which becomes worse in his mind as time goes on. He refuses to marry May to save her from sharing his fate but simultaneously asks her to wait with (and for) him. John is too self-absorbed with his own fate to really open up to May. Of course, "May" is symbolic of spring as well. However, you should also note that her name, "May," implies possibility: a possibility of living fully that John does not see but one that May does.
As John comes to believe that his event will be terrible, we could say it is more symbolically a "fall." The word "fall" connotes autumn as a time leading to winter and as the end of the growing season. The "fall" also connotes the Biblical "fall" of Adam and Eve, which was a fall from grace into knowledge. John was so occupied with himself that he did not allow himself to "fall" into the knowledge of his self-imposed oppression. He also did not allow himself to "fall" in love.
Note the use of "spring," "sprung," and "fall" in the story. Although spring usually signifies a beginning and something good, while autumn usually signifies the end or death, James plays with these terms and underscores John's own confusion.
It had sprung as he didn't guess; it had sprung as she hopelessly turned from him, and the mark, by the time he left her, had fallen where it was to fall.
There is a lot of wordplay, so it is hard to give a conclusive interpretation. One conclusion is that "falling" in love, something John avoided, might have given him a kind of rebirth (spring) and saved him from a lifetime of anxious waiting.