Actually, when this poem was first published, it didn't have a title. Longfellow wrote it as the preface to an anthology of poems called The Waif. In that book, it was just called "Proem," which is a generic title that means the same thing as "preface" or "prelude."
Later on, when the poem was reprinted, it was referred to by the first four words of the opening line. This is a pretty common approach to naming untitled poems. Still, we think it works well here. "The Day is Done" captures the peaceful thoughtful mood of evening. That's a major aspect of this poem, and it gets us ready for all the relaxed nighttime thoughts we're about to read.