Celestia reluctantly accepts the invitation to take a brief vacation, and leaves the sun to Luna's care. What the lunar princess does with the responsibility is anything but responsible, and leaves her with the task of policing the timeline on her own. The result is tragic, but does create some entertaining possibilities.
Well, you got my attention. Let's see where this goes...
This is a mistake, Luna. Cellys gonna be mad.
Oh, my.. to be contradicted by canon in so short a time...
Whoops!! Paradox.