The Mississippi River along Missouri, Illinois, and Arkansas sometime in the 1830s-40s
Slavery is legal. Everyone drunk. And you'd better not touch any rattlesnake skins, because you'll be sure to have bad luck.
Welcome to the South, circa twenty years before the Civil War.
And this isn't the gracious, refined South where everyone is drinking sweet tea and wearing giant dresses. It's a backwater south, full of uneducated, superstitious, and misguided hicks who say things like this: "Deed you ain't! You never said no truer thing 'n that, you bet you" (12).
But we do also see that people can be good and kind: the Grangerfords take Huck in right away; Mrs. Judith Loftus tells Huck—who's a stranger—to "send word… and I'll do what I can to get you out of [trouble]" (11); and Aunt Sally welcomes Huck like a long-lost child instead of a boy who isn't even really her cousin.
And there's real beauty in this South, too. When Huck is on the river, he responds to this natural beauty: "There was freckled places on the ground where the light sifted down through the leaves, and the freckled places swapped about a little, showing there was a little breeze up there. A couple of squirrels set on a limb and jabbered at me very friendly" (8.1).
So what's Twain saying with this setting? Well, like a lot of people who write about the South—like William Faulkner—he seems to see its good and its bad sides—and, more than that, he seems to think that it could change.