In Emily Dickinson’s poem “If you were coming in the fall,” what would the speaker do if her beloved didn't return to her in the fall?

Waiting for your beloved in the uncertainty of his or her return could be the most tormenting experience. The speaker is undergoing precisely the same agony. She’s not sure when her beloved is going to come back to her, or if he’s really going to return. Nevertheless, waiting won’t be a problem for her, however long it might be. What she asks for is the mere assurance that her beloved will come back. With that promise, she can happily spend the time in his absence by musing over the day of their union.

In the first stanza, the speaker says that if her beloved assures her that he will be back in the autumn, she will brush off the whole of summer in the same way as a wife does a fly. The analogy of shooing a fly away with spending a season is uncommon and interesting.

She is suggesting that spending the period of summer in the absence of her lover would certainly be unpleasant and difficult. Nevertheless, the assurance that he'll be back in the fall will be enough for her to spend the period with a smile. She says,

IF you were coming in the fall,
I ’d brush the summer by
With half a smile and half a spurn,
As housewives do a fly.

The poem is about true love that’s not going to change or die with time. It’s also about the agony of separation from the beloved, and the painful undefined longing.

The speaker needs the support of her beloved’s promise that he will return to her in a definite time limit . If she has it, she won’t mind waiting, even if it's for centuries. We feel much pity for the strong-willed speaker because she is denied the only thing that she seeks.