Talk:Afflictions and illnesses/@comment-27040400-20151008092533/@comment-26268292-20151008214007

^ Actually, Celestia challenged Luna to take her job for a day because Luna thought it was easier than hers. Celestia was perfectly fine, in fact, she spent all the day at the spa!

I don't think there is an official comic about either of them falling ill nor replacing the other because of that.