Before her descent into madness and her rebellion as Nightmare Moon, Luna lived entire lifetimes as a mortal and traveller amongst the ponies of Equestria.

Restored to her true self, Luna knows she must return to her cyclic life to remain whole. As preparation she chooses to reacquaint herself with the lives of her subjects in this modern age, and in the guise of Sable Moonshine, unicorn scholar and distant friend of Twilight Sparkle, she travels to Ponyville to spend time with Twilight's companions. Despite the 'exciting' month of astronomical observation and scientific research planned by Twilight, Luna nevertheless finds time to work a day at Sweet Apple Acres. And then another.

And another.

Applejack, meanwhile, is pleasantly surprised by Sable's dedication and enthusiasm for hard work. She's less pleasantly surprised by Sable's enthusiasm for Applejack.