On average it takes sixty-six days to form a new habit.
It took Celestia a millennium.
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For the first few hundred days, she cried. Celestia did not leave her quarters, staring up at the sky. Her sun blazed back down at her, shining on as if there weren't a single thing wrong in the world. Outside celebrations raged – the evil Nightmare Moon had been banished. She was gone. No longer would they live in eternal night. They bathed in the sun, neighing and bucking and just living as if they hadn't done so for years.
By day four hundred and eighty-five, Celestia felt she could finally face the moon again. Every time it rose, the foreign pull going against the fibre of her very being, she would flee. Celestia could not look up at the Mare in the Moon. They no longer celebrated Nightmare Moon's defeat. Some of the older mares would talk to their children of the tale, a fable now blown out of proportion. Hardly any remembered that that creature, donning heavy black armour and wreathed in smoke was their Luna – their Princess.
Day thirty-five thousand exact, Celestia began pleading with Luna to come home. The moon turned away that night.
Seventy-three thousand, two hundred and seventy-four, Celestia stopped begging. She stopped approaching Luna with tears and pleas. "Luna." And the creature ignored her. "Sister." Celestia tried again. Nightmare Moon snorted. What use was there? Luna would not hear her pleas. "We can do this together. You and me. I need you here." But there was nothing.
Day one hundred and twenty-five thousand, three-hundred and two marked the last night Celestia spent staring up at the moon. She was a wreck – a shadow of her former being, consumed by the envious creature that lived in the sky above her head. None of the ponies would know Celestia as she had been, as none of them truly knew Luna. That needed remedying.
From that day on, Celestia became the true Princess of Equestria. She put her heart into it. She would raise the sun and moon, and it started to come to her as naturally as anything. Her energy was drained, and she rarely had time to sleep, but Celestia didn't care. This was her Equestria, and they were her ponies. She had banished Luna for them, and if giving up her free time was the next sacrifice she made, then so be it. She would go through towns and speak with the ponies, bring the sun into their personal lives as easily as she did the sky. Celestia would be the Princess her country needed.
Nopony needed to know that, at night, she would sometimes still beg and cry.
It was on day three hundred and sixty-five thousand, two hundred and forty-two was when Luna ceased to exist. And not only in the memory of Ponies, for generations had passed since that day. Even old wives tales faded from view, and the story of the Mare in the Moon only existed in thick tomes buried in the back of the Canterlot Royal Library. Celestia cast one more look up at the sky, and she no longer saw Luna's face there. She saw the steely, unforgiving expression. It was only Nightmare Moon. That's what she told herself every night. It wasn't Luna any more. Luna did not exist there.
And, as Celestia prepared to step out and greet her loyal subjects for the Summer Sun Celebration, Luna existed.
She was not a solid being, but a spectre, and Celestia wasn't even sure that this was her sister. It could simply be a fantasy – a nightmare. But Luna – the mare in the moon, Nightmare Moon – let out a quiet whicker in her ear. Celestia felt her skin ripple, a shudder chasing down her spine.
This was real.
"Sister."
And everything she had worked for was destroyed.