First Fig Setting
You can imagine Millay standing up at a party (probably on the table, and probably sometime around four in the morning) to bellow out this poem to an awe-struck audience. The poem itself doesn't give any indication of where it's set. We don't know, for instance, if the speaker is addressing a family reunion, a high school reunion, or just a bunch of strangers on the street. What we do know, though, is that the speaker is commanding everyone's attention. Whether you like her or not, you're drawn into her address. And, because she's addressing at least a few people, we're guessing that this is a poem intended for a crowd.
Then again, since the entire poem is focused on a single metaphor, it could be that this is a "speech" intended only for the page. What if the speaker can't get up the nerve to declare her brazenness aloud? The poem wouldn't read any differently if it were an internal monologue, sort of like the voice-overs in Girls.
That's why we say that the poem takes place between the mind and the world. With no clues in the poem itself, it remains delightfully (and maybe even frustratingly) ambiguous. Where does it take place? You decide.