Its a Friday night and Celestia had been invited to a party by one of the most popular ponies in her class. At that age, social climbing among her peers was EVERYTHING and since she was still a blank flank, attending that party and making a good impression on Star Shine would really get her status rolling. The way the week was going, Celestia was certain that nothing would get in the way of her going to that party. Until...
"Celestia, your father and I have to fix the rings around Saturn tonight and we need you to watch Luna while we're gone."
So here we are on a Friday night. Luna is estactic to play with her big sister, while a disgruntled Celestia is certain that mum and Dad planned to fix Saturn's rings on this very night just to annoy her. Her social standing probably took a downfall, Star Shine was probably never going to invite Celestia to another one of her elite parties again, and there was probably no magic in Equestria to turn this situation around. And now Luna needs a new diaper.
"Ugh, I hate changing diapers..."