Time can be a cold, bitter mare, for time causes ponies to forget.
It took nearly six hundred years for them to start. Those who had seen the moon hang unnaturally in the sky were long since dead. Historians began to write of the threat of everlasting night as a legend rather than fact. Mothers told their foals and fillies tales of Nightmare Moon to scare them to sleep.
And slowly, they began to forget Luna.
But not Celestia.
Her dreams replay everything. Every sign she missed, every word Luna spoke, every tear that fell from both Luna's eyes and her own.
Many night's she cries to the night, begs and pleads to whoever is listening out there to allow her to forget.
But time can be a cold, bitter mare, for she causes ponies to remember.