“Celly, what are you doing?”
Celestia put down her quill in annoyance. Luna had interrupted her train of thought. She quickly shut the scrolls she had been writing in them, shoving them all to a corner, revealing the dark wood of the desk that matched the ornately carved walls and ceiling of this particular study.
“Working,” she said unpleasantly, hoping Luna would get the message to leave her alone.
“You're not supposed to be working, you know. This is your vacation time,” Luna emphasized.
“Yes, but this is different.”
“How so?” Luna cocked her head curiously.
“Nopony can know about this except you and me,” Celestia said patiently. She wasn't actually feeling patient in the slightest, but a thousand years of being the sole ruler of Equestria and thus being responsible for the fate of the entire nation 100% of the time tended to do that to a pony.
“Are you talking about that time when you and Mr. Cake--” Luna started to ask but was cut off by Celestia.
“I thought we were never to speak of that again,” she said quickly. “Anyway, it's not that. It's about Twilight. About her becoming a Princess.”
“What about it?” Luna cocked her head. Twilight had always been considered as the most likely candidate for a future Princess ever since she was able to wield the Elements of Harmony to defeat, well...her. And ever since Twilight had succeeded with the task of protecting the Crystal Empire and banishing Kind Sombra for good, she and Celestia had known that Twilight's time would come soon. What they could not agree upon, however, was how soon.
“I'm just not sure if she's ready yet.”
“That's why we agreed to wait for a certain number of moons before giving the spell to her to complete,” Luna said.
“And that time is up, as of next week,” Celestia said. “It's just—there's so much I have to teach her still. Even if we would have had this week, I don't know if it would have been enough.”
“She already knows so much,” Luna said. “In plenty of aspects, far more than me. Just look at her politics! Or her contributions to scientific research!”
“Yes, but--”
“It would be fantastic to have Twilight a Princess!” Luna continued. “Ever since I've known her she's shown exceptional leadership qualities, whether with her friends or with all of Ponyville. She's already the most powerful unicorn in all of Equestria; she would really be able to utilize the Alicorn magic that comes with Princesshood. She's protected Equestria from danger for countless times. And besides all of that, she's a great pony! Overall she's the perfect candidate! If you ask me, she should have been made a princess sooner!”
“But there's still so much I need to teach her,” Celestia said softly, her voice cracking.
“You needn't worry,” Luna said comfortingly. “Besides, after she becomes a Princess, you can still offer her advice.”
Celestia bowed her head, subtly hiding her facial expression. Luna, not knowing what to say, stayed silent. It was clear that her sister had hardened over the years that she was gone and had probably not cried in centuries. Luna nuzzled her sister softly before Celestia took a deep breath, attempted to collect herself and took a step back.
“Over the years, I feel like the bond between Twilight Sparkle and me has continuously grown,” Celestia said softly. “I always tried to be there for her, since as she grew older and enrolled in the Academy, her parents and brother increasingly weren't. I would teach her magic, let her sit in some political discussions, educated her about the world. I watched her grow from a young, excited filly to a wonderful mare, and even after she moved to Ponyville and found friends, we still stayed in close correspondence.”
Luna nodded, but Celestia did not see the gesture, turning her head away from Luna towards the window.
“We still met up every other Tuesday in Canterlot,” Celestia said quietly. “She would take the train to Canterlot and meet me for tea and cookies in this very room. We would discuss friendship lessons and her studies and politics. And yesterday--” Celestia's voice cracked slightly and she took a deep breath, begging herself not to cry. “Yesterday was one of those Tuesdays. She was so busy preparing for the Parliament session that she forgot to tell me she couldn't make it in time.”
This wasn't just about tea and cookies. Luna opened her mouth slightly in realization. “It would have been the last time. The last time before...” her voice trailed off awkwardly.
“Ever since—“ Celestia choked slightly on her words, “--a while ago, I had been dreading the day that Twilight would outgrow me as well. But now it seems to already have passed.”
Ever since when? Luna decided not to pry, she certainly didn't want to make her sister feel any more upset. “You needn't worry, dear Sister,” Luna said. “Twilight respects you just as always, and her new duties will only bring her closer to you. You will have so much to teach her about being a Princess, she so much to learn from you.”
“But it won't be the same,” Celestia whispered, bowing her head. “I'll miss my most faithful student.”