Celestia often thought back on memorable events in her life when she had the chance to, much like any other pony who has the time to look back on their lives. Of course, the sense of scale of what constitutes as 'memorable' becomes slightly different when one has lived to see a thousand years.
But, there were some that stuck in her mind. Besides the obvious ones the history books could name, there was the time when some lesser noble who's name was now lost from history attempted to obtain the Neightcronomicon, in order to usurp her rule. He actually managed to track down its location within the castle, through bribes and magical scrying, only to flee Canterlot in shame when he realized that the only copy left in the world was being used to hold up a leg on Luna's favorite reading desk.
There was also the time when she visited dragon lands, to see the new high king being crowned. She had to politely refuse a marriage proposal from a high noble who was several meters long and had a name that was older than Equestria itself, and Luna had to deny the gift of a fur cloak, which, besides being a faux-pass in pony society to wear hide, was made from the entire pelt of an Ursa Major and could have been draped across Canterlot itself.
All in all, besides five attempts at her life and an rather embarrassing incident when one of the bat-pony guards was caught snuggling with one of the younger drake courtiers, the king was crowned successfully. As a token of good will, he provided the egg of one of his brothers, as an offering of peace. Luna's offering of Blueblood the Tenth as a trade for the egg was greeted by the dragon's as a show of good humor, but the dragons were probably unaware that the prince had two enchantments on him; one casted by herself to prevent him from saying anything rude, and another that her Luna placed under him, to prevent him from screaming.
Yet another day she tended to review in her head every so often took place roughly five hundred years ago, when a group of ponies who called themselves the Daughters of Nightmare Moon attempted to assassinate her, only to be foiled by another group called the Sons of Sombra, who were also trying to kill her. For whatever reason, both groups assumed that the other was trying to stop the other and they wiped each other out within a weeks time. While the resulting mess caused by two groups of secretly evil unicorn nobles revealing their evil intentions at the same time and subsequently trying to destroy the other wiped out three noble bloodlines and required one of the halls of the castle to be rebuilt, Celestia still remembered the looks the two cult leaders had when they faced each other, giving that they were husband and wife.
Dark humor was her sister's forte, but Celestia couldn't say that she didn't take humor when she found it.
She also took to traveling a bit around the world for a decade, during Equestria's early years, visiting lands to formally announce her rulership and to alert all the peoples of the world of her existence. She spent a day in a Zebra king's palace, sat in a peace talk between the Hippos of Denile and the Rhinos of Horneo, and grazed among the Gazelle and Giraffes. She had toured Roam, and ate her own weight in pasta between Fleece and Prance. While evading Minotaur marauders and Satyr bandits, she met with Griffon archons and Pegusi helots alike on her way west, finding the former surprisingly agreeable and the latter distressingly xenophobic and brutal.
She then took a year-long journey down below the surface of the world, both to avoid the cold of the northern steppe and to see the strange realms that her sun did not touch. While the Diamond Dogs of Pawland took great care to pretend that they did not want to end her life, the king of the Cavern Cats of Purrisa defecated on her gift of precious jewels when she visited their dusty court. As if in a attempt to appear even more hostile, the Changelings of Tir Na Bog sent a squad of very heavily armed diplomats to 'greet' her and to give her false directions, which led her so down below the surface of the world she felt the heat of the core of the world itself.
Regardless of how she felt on the creatures of the Underdark, the lands where her sun were a constant present were far more inviting, and she personally found that the Camel sheikhs and Horse sultans were equally hospitable, even though Celestia suspected that their good will had to do with her ability to control the sun. The lands of the Elephant rajahs and Naja ranas were splendorous to behold, and even though she did not personally agree with the former's treatment of their water buffalo subjects and fervently objected to the latter's wars with the Garuda and Rakasha's, she took notes from how they ruled their lands all the same and enjoyed their general affability regarding her existence.
The lands of the steppe bison were hard and uninviting, much like the steppe bison themselves, but she made her existence clear to the tribes all the while. In the lands of the Markhor shah and Centaur khanates, her arrival was greeted coldly, but understandably so; they were both still wary about ponies due to the 'diplomatic incidents' regarding Tambelon and the destruction of Midnight Castle, and she herself took pains to smooth things over with the leaders of both groups.
The fact that her journey ended with a Kirin shogun ordering his troops to fire at will when she was in sight of his domain didn't make her especially upset, but finding out their emperor claimed that he controlled the sun did. A quick magical message to Luna to lower the moon for a few moments was all it took to solve that little mistake, of course. She stayed around to oversee the crowning of the new emperor, to make sure the younger one wasn't anything like his forefather.
But, for all the sights she has seen and all the things she had heard, there were still times when she could be caught off guard by her very own subjects, the gentle-minded pastel ponies who she loved with all her heart, body, and soul. The ponies she traveled the world for the sole purpose to not be surprise by their capabilities.
Such as earlier today, when her favorite student flung the door to her throne room open, mane mussy and eyes wide, half-screamed out something resembling "Discord convinced a filly to be his student!", let out a little wheeze for air, and slumped on the ground.
Besides Twilight apparently forgetting that she could simply have Spike send a letter, or teleport, for that matter, the knowledge that the most powerful being in Equestria's borders apparently had picked out a heir of sorts caused Celestia to suddenly be very, very aware that something of deep concern had occurred, and it was, in fact, partially her fault.
Ducking slightly so that her horn did not mar the door, Celestia quickly took note of the homey little dwelling of the bearer of Kindness. She found herself in simple wooden house, very homey and very clean.
There was also a small pink filly with a little bejeweled tiara sitting on a couch in front of a tea set, who was sitting very, very still.
"Fluttershy, would Discord happen to be here?" she asked slowly, seeing that the filly was stunned by her presence.
Celestia noted that a look of understanding passed over Fluttershy's face, and that she didn't need to be told why she was here.
"No, Princess." Fluttershy replied lightly, before taking a quick glimpse outside, where a chariot, two royal guards, and a very concerned Twilight Sparkle were parked directly outside the house, "I don't mean to be a bother, Princess, but I was on my way to see Twilight. I am sure that Discord will be here soon, though."
Celestia smiled warmly at Fluttershy. "Oh, there is no need to apologize to me, Fluttershy. I am sure Twilight would like to see you."
Raising her head slightly, she turned to look at the small pink filly with the mug in her hooves, who was staring at her wide-eyed.
Lowering her head, she looked back at Fluttershy, who, after a few moments, gave one of the tinier nods to ever occur in history before walking out of her house.
Celestia, without saying another word, slowly trotted into the living space set in the main room and settled on simply standing on all fours, because when one's job consisted of sitting on one's butt for eight hours a day one tried to stand up as much as possible for the other six hours one was awake for to enjoy.
"Ah, hello, my little pony." she said to the filly, who was still giving her that awful staring look, "May I ask for your name?"
"M-my name is Diamond Tiara, Princess."
"Diamond Tiara." Celestia repeated, looking the filly over once more, memorizing the name. "Little one, you do not have to call me 'princess'. You can call me by my name, if you wish."
Diamond Tiara's mouth opened and closed a few times, like she wasn't sure if she wished to respond or not, before finally taking in a small breath and shifting the cup between her hooves.
"Yes, Celestia." she said slowly, like the name was an unfamiliar one. Celestia nodded slightly, in approval. It was a start.
Eyeing the tea table and the abandoned cup of tea on it, the princess frowned lightly.
"Do you know why I am here, Diamond?"
The pink filly gulped lightly, and Celestia felt a ting of guilt.
"My dad hired Discord to be my tutor."
Celestia just gave the filly a little smile of conformation.
"Well, you must be quite the filly to catch his eye, then."
Celestia paused, looking over Diamond Tiara once again. The filly's name wasn't one she recognized, but she wore a rather expensive piece of jewelry and had a odd glint in her eyes that was vaguely familiar to her. While she didn't believe that Discord cared a single whit about a pony's background before he tormented them, Celestia supposed that it would be prudent to at least know who among her subjects would allow their daughter to be taken under the patchwork wings of the Draconequus.
"Would you mind telling me who your parents are?" Celestia asked sweetly.
Diamond Tiara, to her slight surprise, suddenly looked eager to talk, and for a few moment's she resembled the epitome of innocent, youthful joy. "My dad's Filthy Rich, the pony in charge of Barnyard Bargains."
Celestia took her time pondering what that really meant, on the grand scale of things. The filly and presumable heir apparent to one of the wealthier companies in Equestria being assisted in education by the Spirit of Chaos spelled dark times for the financial honesty of her realm. But, the question was, did Discord chose the filly because of her status, or did he see some 'inborn talent' he could nurture?
No, why would she think otherwise? Of course he would chose a filly who he thought he could control directly. He had nothing to gain from politics; he didn't have anything to gain from ponies, really, except from the entertainment he could gain from them.
With that in mind, she looked at Diamond with a slight wariness.
"Diamond, what about your mother?"
The filly's apparent good cheer seemed to diminish, and for a few moments Celestia believed that she had hit a sore spot.
"My mom's in Manehatten, Princess." the filly replied, in a way that made Celestia feel a wave of curious familiarity.
"How long has she been there, if I may ask?"
"She's been staying with my aunt for a month, but she's going to visiting Ponyville soon." the filly said matter-of-factually, in a way that made Celestia, the Diarch of the Sun, slightly worried.
It was her eyes, Celestia realized, and the way she talked. Discord couldn't have possible have known that filly had such an unfortunate family, unless...
...whatever drew him to her was due to the symptoms of the filly's situation, not the situation itself.
"Is that so? You're aunt and you're mother must be close." Celestia replied, encouragingly.
Diamond shook her head. "No, my aunt Suri and my mom don't really like each other, but my aunt makes clothing and my mom sometimes works with her to keep herself busy."
Celestia's smile was painted on her face by the time Diamond was finished speaking, and it wasn't entirely due to what the filly was saying. It was due to who the filly reminded her of, and what Discord could possibly be looking for in a student.
The filly's posture was just right, as was her tone of voice. She had the same sort of self-confidence, the same look in her eyes; even her eye's were just a shade darker then teal. Discord couldn't have known, but if she was anything like her, well, her 'special talents' would be much appreciated by the bringer of chaos, for all the wrong reasons.
Celestia suddenly became aware that she was, in fact, staring at the filly, and decided it was time to leave.
"I do think it is time for me to go." she said quickly, fluffing her wings as she did. "Before I leave, little one, has Discord taught you anything yet?"
"No." the filly said easily. It was a lie, Celestia knew, but it took even herself, who had seen generations of sly nobles come and go, a few good moments to recognize that. Diamond Tiara was a very good liar, which reminded Celestia of the only other pony around her age who was capable of lying so smoothly.
But, Celestia didn't betray the fact that she knew that the filly had lied, and simply nodded, smiled warmly, and headed for the door.
"Little one, I think it might be prudent for you to head home. I don't believe Fluttershy will be coming back home soon."
She said that with a smile, and a quick glance outside showed her that the carriage was still parked outside, waiting for her to leave.
"Yes, Princess." Diamond meekly replied. "Am I going to be in trouble?"
Celestia didn't actually respond to the filly's question, as while she was exiting the house, she was busy wondering how Discord managed to track down a pony who, for all intents and purposes, reminded her of no less then a young, hornless Sunset Shimmer.