*AN: This takes place almost immediately after the Luna issue of the Micro Series Comics. Hopefully, I have explained enough in the story for you to be able to understand even without having read it, but I thought I would still let you know. Also, this is fanon I came up with on the fly, and the first thing to even remotely make sense for there being another side to the Celestia- Nightmare Moon story, so if it's not good, politely say so, and perhaps help me come up with a better idea!*
An exhausted Luna was en route to her quarters after her shift, having no sleep at all in between this one and her last. The morning before, she had told her sister, Celestia, that she could handle both day and night, as she had done for so many moons. Though she did prove to be successful at this task, it was at no small consternation. Luna was more than glad that from now on, she would handle only the night. As she approached her room, she passed Celestia as she was headed to raise the sun.
"Good morning, Luna!" She cheerfully greeted.
"Glad that somepony got some good rest." Luna replied. "I still don't see how you manage. My one time at handling the day was hectic, but you handled both day and night for a thousand years!"
"It wasn't easy," Celestia admitted. "It almost took all of those thousand years to master. I had never realized just how wonderful it was to have a sister to help me."
"I cannot apologize enough for only adding to your workload." Luna looked at the ground.
"I should apologize, too." Celestia put a hoof on her sister's shoulder. Luna looked up at her sister in shock.
"You?" She asked.
"Yes." Celestia nodded solemnly.
"If it's for banishing me, I completely deserved it."
"No." The white alicorn shook her head. "It is for much more than that."
"What, then?"
"One could partly blame me for your becoming Nightmare Moon."
"What? How?"
"I messed with our destinies." Celestia turned and gazed at a window, through which one could see the Equestrian flag. "Do you know what the two ponies on our flag represent?"
"But of course," Luna replied. "They represent us, and harmony."
"That, and something else. Balance."
"Balance?" Luna looked closer at the flag.
"Yes. You know the concept of Yin and Yang?"
"I think so. Two complete opposites, each with a little bit of the other, combined to make a perfect, balanced whole."
"Correct." Celestia pointed at the flag. "One could say the silhouettes on the flag are arranged in the same way as the symbol of Yin and Yang."
"I guess I could see that…" She squinted to see better. "But what does that all have to do with my becoming Nightmare Moon?"
"Complete opposites…" Celestia began. "Each with a bit of the other. Our parents strived for such harmony in Equestria."
"And…?"
"When we were born, our parents saw that I would be a bright, kind leader, and that you would be… well, you. They thought I best represented the day, and you best represented the night."
"Hence our positions" Luna was starting to consider postponing the story and going on to bed. This was going nowhere!
Celestia shook her head again. "Not at first. Going with Yin and Yang, they would have had you rule the day, to represent a little bit of night under the sun, and I would rule the night, as a bit of sun in the night."
Luna was slack jawed, her eyes wide. "B-b-but what happened?"
"Growing up, I learned of our future roles in Equestria. It didn't sound right to me. I feared you would react to the day's chores as you did yesterday, stressful, and frustrated. So, despite our parents' wishes, I had you sleep during the day, gave excuses for your absence to our parents so I could rule during the day, making you rule the night as if to make up missing your shift. Eventually our parents decided just to switch our intended roles, since it was obvious we did very well in the ones we assumed."
"How come I never heard of this?" Luna wondered. "And, why do you consider this a reason to blame you for my tyranny?"
"I never told you, and always gave you reasons for sleeping in the day, and staying up in the night." Celestia explained. "As for my blame… I did what I did because I thought that it would be best for you. Then, I saw how much you enjoyed the short glimpses of ponies playing in the day, and wished to join them. I even took notice of your jealousy towards me for the attention I received in the day.
"But still I did nothing, for I was still certain that that would be best for you. By the time it got really bad, and I started to reconsider, you had turned into Nightmare Moon, and there was nothing else I could do but banish you for all those years. I considered taking on responsibility for both day and night my just deserves for forcing you down the path that led to your being alone for so long."
"Oh, Tia…" Luna wiped away a tear forming, and went to catch those that were beginning to drop from her sister's eyes. "I don't blame you in the slightest. If anything, I thank you. I adore the night, as nice as the liveliness of the day can be. You had no control of my jealousy, or my choice to be the host of the nightmare forces."
"I-" Celestia started. "I still could have-"
"It is no matter what could or couldn't have been done." Luna stamped a hoof. "It is in the past. And, inside my jealousy, has always been admiration for you. You are so fit for the day: so patient, so kind, and caring. I admire you even more now that I know this. You gave up the gorgeous, easy to rule night for me. You risked the wrath of our parents to spare me. All that you did which I thought was for your own glory: taking the day shift, making me sleep through it, etcetera, were all done out of your love for me."
"You… you aren't angry?" She looked up at her young sister finally.
"Not in the slightest, dear sister." The two sisters hugged. "You did not mess with our destinies. You helped us fulfill them. I mean, imagine me with a sun as my cutie mark!"
Celestia laughed. "Thank you. And, I should probably let you get your rest now."
"I almost forgot how tired I was." Luna chuckled. "Well, good say, sister." She turned back to the hall.
"Sweet dreams, Luna." Celestia smiled, and headed towards the balcony to raise the sun.