One day. That is all the love the speaker of Donne's poem has been given, and he is already scripting the speech that will end it. The title promises a poem about woman's constancy, but "constancy" here is a joke, since it has lasted exactly twenty-four hours. The opening line makes the sarcasm plain: "Now thou hast loved me one whole day." The word "whole" is mock generosity, as if she had performed a marathon.
What follows is a list of the excuses the speaker expects to hear tomorrow, each introduced by a weary "Or." She might backdate a new commitment ("antedate some new made vow"), so that her betrayal looks like an earlier loyalty. She might claim that "we are not just those persons, which we were," a philosophical dodge that treats the self as so changeable that no promise can bind it. She might say that oaths sworn in fear of Love and "his wrath" can be broken, since Love is only a god and gods can be defied. The most elaborate excuse is a chain of resemblances. Death dissolves true marriages, lovers' contracts are "images" of marriage, and sleep is "death's image," so a lover's promise ends when the lovers fall asleep. The logic is absurd but ingenious, and the speaker clearly enjoys quoting it. His mockery shows the wit of a man who has heard it all, or imagines he has.
The form helps the performance. Most of the poem moves in iambic pentameter, but the lines swell and shrink unpredictably. Brief lines such as "Or say that now" and "Which I abstain to do" land like a raised eyebrow or a shrug after the longer ones. The rhyme is mostly in couplets, with one triple (*were*, *fear*, *forswear*) and one pair of rhymes wrapped around another (*untie*, *those*, *unloose*, *justify*). That wrapping suits the tangled "image of an image" argument it contains. The poem sounds like improvised speech with its tidy pairings, but the shifting line lengths keep it from settling into a pattern.
The final excuse turns the paradox up a notch: she must justify "having purposed change, and falsehood" and so "can have no way but falsehood to be true." Her only way of being faithful to her new intention is to be unfaithful to him. Then the speaker calls her a "vain lunatic." The insult reaches back to *luna*, the moon, the old emblem of changeability, so it quietly accuses her of being inconstant by nature. This is my inference from the word's root rather than anything the poem states, but it fits the poem's argument well.
The ending is where the poem becomes more than a put-down. The speaker claims he could "dispute, and conquer" every one of her arguments, but he will not, and his reason is the sting: "For by tomorrow, I may think so too." Suddenly the man who has been mocking inconstancy admits that he is just as changeable. He might be persuaded by her sophistry tomorrow, because tomorrow he may be a different person, exactly as one of her excuses said. The short line before the finish ("Which I abstain to do") works like a held breath, and the long last line releases the surprise. His refusal to argue looks like generosity, but it is really a confession.
That reversal is why the poem matters. It begins by borrowing a familiar complaint, that women cannot be faithful, and ends by turning it on the man who voices it. Both lovers now seem equally fickle, and the title's irony deepens because "woman's constancy" turns out to be no worse than man's. The poem also shows a habit of Donne's love poetry: it argues instead of simply sighing, using mock logic, cheeky tone and a sharp turn at the close to say something uncomfortable about desire. It leaves us laughing, then wondering how much of the speaker's cynicism is really self-portrait.