1) The coronation of the two sisters, the beginning of the Sun and Moon era, Celestia was 22 and Luna 15, Starswirl took care of the royal duties until both of them were ready. Luna felt extremely nervous that day, Celestia comforted her sister the best she could, telling her that not matter what they would be together, always, and would bring a peaceful reign like their mother. Celestia's "official crown" is modeled after her Great-grandma's crown (Princess Platinum crown), since Luna held the title of Princess, hers was more modest. The princesses chose their more casual crowns themselves.
2) The years that came weren't easy, first Discord, and Tirek…and then the Pony of Shadows, who took their mentor with him, and they just arrived from the Crystal Empire after banishing King Sombra. The princesses were exhausted, tired and stressed. Luna felt it more, her subjects were scared of her due to her Dream Walking, she stopped showing up to royal events, she was too tired in the day…it was then when a shadow figure appeared before her. It called themselves "Somnum" and said to Luna that she could feel her sorrow, and they were alike in many aspects. It gained the trust of the Princess. What she didn't know was that Somnum was a force made of pure darkness, a spirit that feed themselves of the darkest wishes of the ponies.
3) With time, princess Luna started to act less more like herself, she didn't lowered or raised the moon in time, hardly even went outside of her room, and in their subjects dreams there was a dark figure with her. This worried Celestia, who confronted her sister,"I'm only trying to do what is best for us""Well, I never asked for this, I never wanted this"harsh words were said, and after that Luna felt worse than ever, Somnum took advantage of this.
"A-are you sure that if i let you do that…you will convince Tia that my night is worthy?" "Of course dear, let me take care of everything, you will be asleep the whole time!!"
4) The night Nightmare Moon was born, Somnum possessed the body of Luna, and refused to let the moon down, no more light would shine in Equestria ever. Celestia felt horrified, she should have done something before this…but it wasn't time to pity herself, she tried to be as careful as she could to not damage Luna's body, but Nightmare Moon was too strong, this is when she used the elements, she was sure they would banish the dark form and only leave luna, but the elements can't be controlled by one person…it was too late when she banished her sister to the moon.
Celestia was devastated, the elements didn't respond to her anymore, she tried everything she could, but nothing worked. With help of the royal astronomers she calculated how long her sister would be trapped in the moon…1000 years. 1000 years of loneliness, and 1000 years to find someone who could control the elements.