The Princess was once asked what the meaning of life was by an aspiring philosopher.
The glorious Princess Sunbutt had one simple answer. Everything, from love and sorrow, life and death, fame and poverty, revolved around butts. But what would a butt be without the pony attached to it? Any pony butt could have a mark of the sun on it, but it took a large mare with a large flank to get their butt off the couch and actually accomplish something great.
While anypony could have a shield or a pizza on their butt, it took ponies who didn’t need a swift kick in the rear to be motivated to follow their set path in life.
It took powerful ponies to get their butts out of bed, to kick some butt as a secret agent or for a butler to butter bread. Ponies were more than their marks or their butts, they were champions of harmony who fought against villains and buttheads alike!
The Philosopher, after figuring out that philosophy wasn't for him and that he should be a psychologist, realized that Celestia was very self-conscious about her butt, and call a pony by their butt day was born.