This story is a sequel to Harmony Unbound
Luna is home again. A new home, in fact; a library in Ponyville. Things are messy right now between her and Celestia, and awkward. After things ended so poorly between the two of them last time, and clearly neither of them is ready to face the other again, she instead found herself a new place in the world.
But Luna is not sad. She has a new life, and new friends to lift her spirits. It's all helped her settle in, settle her past, and finally be happy again. Even if she cannot or does not go back to her old life with Celestia, she's plenty confident she'll remain cheery and content.
Then tickets to the Grand Galloping Gala arrive for her and her friends, and it completely stresses her out. Because Celestia will be there, and she might not be ready, even by then.