((Prompt: The birthday of a legend.))
It was quiet. Too quiet. Twilight glanced around nervously, taking stock of Ponyville around her. Ponies walked the streets, chatting amiably as they made their way to and from their destinations. In the market, merchants hawked their wares while customers browsed through the goods. Birds chirped and flew across the bright blue sky. All in all, it was a very normal, peaceful day.
Too normal. Too peaceful. Where was Pinkie? Twilight suddenly realized that she had not seen the party mare around town in the past week. She had caught glimpses of her darting in and out of the back rooms in Sugarcube Corner, and she had noticed light and noise coming from the window in the upper story late at night, but Ponyville had been mostly Pinkie-less for a whole week.
Twilight swallowed. She did not like to subscribe to premonitions, but she had a feeling like the calm before a storm. Much like old ponies did not need to check the weather schedule because they could feel it in their bones when a rainstorm was gathering, Twilight could tell something big was about to happen. Pinkie Pie was about to happen.
It began with the pink pony wheeling her party cannon out the back of the store. The barrel's caliber had been doubled, and thick, heavy supports had been welded to the sides. With a single-minded focus, Pinkie placed the cannon in the middle of the main thoroughfare and aimed the barrel straight up. She spent a good fifteen minutes ratcheting down all the supports and double-checking their sturdiness as ponies flowed around her, mild curiosity on their faces.
Satisfied that her cannon was set up properly, Pinkie zipped back into the store. Twilight looked at the oversized piece of artillery, chewing her lip. Before she had a chance to investigate it any closer, though, Pinkie re-emerged with a similarly oversized artillery shell balanced on her back. The shell appeared to be made of tightly-packed confetti and streamers. Pinkie unfolded a ladder and climbed up to load the shell into the muzzle of the gun.
Twilight finally opened her mouth to speak. “Pinkie, what—”
Pinkie dropped to the ground and gave the firing cord a swift tug. A deafening boom rocked the entire town. The shell shot into the air, a contrail of streamers behind it as it climbed for the sky. As it reached its apex, it burst. A rain of multicolored confetti and streamers and banners covered Ponyville in a layer of festive decoration.
“IT'S BIRTHDAY!” Pinkie shouted, throwing her forehooves above her head. The denizens of Ponyville blinked in shock as confetti gathered about their fetlocks.
Twilight picked herself off the ground, rubbing her ears. She was certain she was going to go deaf before she turned thirty. “It's your birthday, Pinkie? I thought it was in March.”
Pinkie giggled and shook her head. “No, no, it's not my birthday. Today is the Birthday!”
“The Birthday?” Twilight repeated.
“Yeah! Today's the day we celebrate the birth of birthdays!” A party hat floated down from the sky and landed a little askew on Pinkie's head.
Twilight tilted her head. “You're gonna have to explain this one to me.”
“This is one of the first things you learn in partyology,” Pinkie said, draping a hoof over her friend's shoulders. “See, two thousand years ago, in the Romare Empire, there weren't any birthdays! Ponies just got older, and that was it!”
Pinkie produced a cupcake from—actually, Twilight wasn't sure where she had gotten it—and took a bite. “But then came a pony called Partius Timaeus. He loved parties, but they only happened on holidays. The day he had been born on was right in the middle of that long stretch between Winter Wrap-Up and the Summer Sun Celebration where there isn't really any good partying holidays, so he declared that any day a pony was born on counts as a partying day!”
Twilight wanted to point out that the Summer Sun Celebration wasn't made a holiday until after the fall of the Romare Empire, but she was instead dragged along as Pinkie continued. “Partius wasn't in any kind of position of power to make that kind of declaration, but he just happened to have the same birthday as the Emperor, who thought it was a great idea. So they threw the first birthday party!”
Twilight stopped. “Wait. You said Partius' birthday was between Winter Wrap-Up and the Summer Sun Celebration.”
“Uh huh,” Pinkie said, nodding.
“Pinkie, it's fall right now.”
Pinkie Pie picked up a party hat and patted it onto Twilight's head. “Twilight, Twilight, Twilight,” she tutted. “Didn't you get the message from the story? Any time's a good time to celebrate Birthday!”