We usually refer to this poem simply by its first line, "She Walks in Beauty." But the first line does more than introduce the subject of the poem – a beautiful woman. The first line of the poem (and therefore the title) is an apparently conscious echo of the famous sonnet by William Shakespeare, "Shall I compare thee to a summer's day" (Sonnet 18). Except, of course, instead of comparing the beautiful woman to a "summer's day," Byron compares her to "night." So he's not just setting up a contrast between night and day, he's also setting up a contrast between himself and Shakespeare.
This is a pretty gutsy move, if you think about it – even in the early nineteenth century, when Byron was writing, Shakespeare was generally accepted to be the greatest English poet of all time. Usually, when poets referenced Shakespeare, they did so in an almost reverential way. But here, Byron gives Shakespeare a shout-out, only to turn Shakespeare's simile on its ear and reverse it. Based on what we know about Byron (and check out the "In a Nutshell" for more on that), this kind of gutsy move was entirely in keeping with his general character.