There are things a Princess cannot do. Over the centuries, Celestia has been asked to do those things countless times. And the moments when her ponies need her the most are the ones where she can't do anything.
On the night of her mother's passing, Luna must accept the cold, hard fact that she'll never feel her mother's loving touch again. With no one to else to turn to, she finds solace and a much-needed shoulder to cry on with her big sister.
For nearly every citizen of Equestria, it's a holiday: a chance to get together with friends, gather a few treats, and laugh in defiance of the darkness. For Celestia, Nightmare Night is something else entirely.