Afer being hit by the Elements of Harmony, Sunset Shimmer finally understood just how evil she had been, and wishes for nothing more than to make up for it, and seek out forgiveness as Twilight told her.
The next morning she wakes up a week before she went through the portal to steal the crown, 10 days before the Fall Formal, and decides to try and right some wrongs she has committed among CHS before she leaves for Equestria.