We think "The Day is Done" is all about the power of art. When the speaker is feeling down, the one thing he can turn to is poetry. He wants us to know that books and poems can make us feel better. They can stir our emotions and fill the world with beautiful music. Come to think of it, that's pretty much how Shmoop feels about it too, so this poem works just great for us.
Longfellow uses this poem to both describe what poems can do for us and to actually do it. While he's talking about how poems can soothe the reader, his language is working to calm us and make us happy.
The poem works to build a connection between author and reader. Longfellow literally invites us to read a poem aloud to him, creating a kind of magical connection across the distance of time.