Princess Celestia/Synopsis

Prologue
Celestia was introduced in the prologue as the older one of two sisters (the younger one being Nightmare Moon). She used her powers to raise the sun at dawn while Nightmare Moon brought out the moon to begin the night.

However, the ponies played in the day but "shunned" the night and slept through it, which made the younger one grow bitter, eventually refusing to lower the moon to make way for the dawn. Her bitterness transformed her into a wicked mare of darkness: Nightmare Moon. Celestia reluctantly harnessed the power of the Elements of Harmony and banished her in the moon for 1,000 years, maintaining harmony in Equestria by taking responsibility for both sun and moon.

The day of Nightmare Moon's return is also the 1,000 year anniversary of the Summer Sun Celebration, so Celestia sends Twilight Sparkle to oversee the celebration's preparations, which are being held in Ponyville, and encourages Twilight to "stop reading dusty old books" and make friends, much to her surprise.

Celestia is expected to appear at Ponyville's pavilion, but she goes missing and Nightmare Moon appears in her place instead. After Nightmare Moon is redeemed by the Elements of Harmony, she offers her younger sister a chance to rule by her side again, and she accepts.

In one alternate timeline in The Cutie Re-Mark - Part 1, Celestia wages a war against King Sombra and his Crystal Ponies, leading the royal guards and Wonderbolts into battle. In another alternate timeline in The Cutie Re-Mark - Part 2, the events of the prologue happen with the roles reversed. In a third alternate timeline leaded by Discord, Celestia and Luna are reduced to being Discord's personal entertainment

Prior to ruling Equestria
Celestia described more of her history in The Return of Harmony Part 1. She tells Twilight Sparkle and her friends that before Princess Luna and she stood up the Discord, he ruled over Equestria, keeping it in a state of unrest and unhappiness. Celestia goes on to describe that after seeing how miserable life was for Earth ponies, Unicorns, and Pegasi alike, she and Luna discovered the Element of Harmony and defeated Discord, turning him to stone. Discord's spell is later broken because, as Celestia explains, "Luna and [herself] are no longer connected to the elements". In The Crystal Empire - Part 1, Celestia describes how she and Luna worked together to defeated King Sombra, turning him into a shadow and imprisoning him in "the arctic north". Even further back in time, Twilight's Kingdom - Part 1 confirms that Celestia was an associate of the legendary Star Swirl the Bearded. During the Pre-Classical era, Celestia and Luna were involved in the first defeat of the evil centaur Lord Tirek, sending the villain to Tartarus.