Princess Twilight Sparkle walked into the Ponyville branch of the Carousel Boutique fashion chain, holding a scroll in her magic. There was a mare she needed to see urgently, a seamstress for there were only a few weeks until the Grand Galloping Gala and she needed a dress to wear for the occasion. She had a perfectly good outfit in her wardrobe, but this was her tenth Gala as the ruler of Equestria, and she wanted something special to commemorate the milestone.
"Twilight!" a pale-coated unicorn called. Though there was a grey streak in her glamorous curled purple mane, Rarity was just as beautiful as when they had first met, in Twilight's opinion.
"Rarity! It's good to see you! How are the little ones?"
"Oh, Sapphire and Diamond are doing wonderfully. Sapphire's being just like the Cutie Mark Crusaders were back in the day, trying to insist that she has to go to the School of Friendship. She'd do far better at the Gifted School for Unicorns, if you ask me. She knows some spells that I can't even cast, even after being under your tutelage. Now, though, I have a feeling you're here for something other than small talk, by the look of that scroll. Come into to the back room."
Rarity led the way, and Twilight walked slowly in order to admire all of the beautiful gowns on sale. They really were exquisite, but Twilight already knew exactly what she wanted to wear.
They sat on couches, Twilight occupying an entire sofa due to being so... big. That was one of the complications when it came to clothes; she was a fully grown alicorn, which meant she had longer legs, huge wings, a larger torso, a prolonged unicorn horn and a giraffine neck.
"So, what is it, dear? Saving Equestria business?"
"Of course not! It's something much less important, but still urgent. Since I've been in charge of Equestria for ten years now, I'd like a special dress to commemorate the occasion. One particular dress you've made has taken my fancy."
"Oh? I presume you have a picture in that scroll."
"In fact I do." She unrolled it and Rarity took a glimpse at the photograph that had been stuck to the page with sellotape.
It was a picture of Twilight from many years ago. A teal chest piece, like the ones worn by all the alicorn rulers, sparkled. Twilight's actual wings in the picture were covered by turquoise silk, replaced by a pair of small white wings attached to the gown at the top of her back. The turquoise fabric covered her flank before it ended and a length of cerulean silk covered most of her legs. The bottom of the dress was once again silk, this time in a royal purple hue. The dress was jaw-droppingly glamorous, one of the most elegant dresses Equestria had ever seen.
It was also the one dress Rarity had vowed never to make again. The Princess Dress.
"Darling, this dress has been discontinued for nearly fifteen years!" she gasped. "I could not possibly make you something that old!"
"Why not? Your dresses are getting better by the day. I'm sure you could make it again for me, Rarity."
"Darling, do you want to hear my real opinion?"
"Go ahead." Twilight was not taking the hint.
"Dear, this thing is the most unsightly dress I've ever sewn, and I am quite frankly disgusted that you have asked me to sew it again for you. Has your memory faded? That ugly thing in the picture you have shown me today made me want to abandon my dream of becoming one of Equestria's top fashion designers, which means I would never have got where I am now. Go find somepony else to make you a dress, and as horrifying as the design is, make sure nopony steals it from me. I want that picture to be burned, just like all the dresses."
"But I -"
"There are no buts about it, Twilight. Get out. I don't care whether you are the socially awkward unicorn I met nearly two decades ago or the ruler of Equestria you are today, but the fact that you are insensitive enough to suggest that I bring back this design is astoundingly rude of you. Leave my store, and don't come back until all known traces of that thing are destroyed."
"I -"
Twilight knew she was in the wrong, deep down. She forced herself to leave the room and left Rarity to remember when she first set up Carousel Boutique Canterlot.
She had created a collection of designs for the grand opening, all based on the royals. Twilight had worn the Princess Dress. It had been successful, but too successful. Nopony wore any of Rarity's other designs, and it was like working in a factory. Creating the same garment over and over again was not what she had wanted to do since she was a filly. She had decided to close the store and stop making the dress, but that was when everypony else had finally noticed that they could buy outfits other than the Princess Dress. All in all, it had turned out fine, and she now had three stores in Canterlot, but the memory of the Princess Dress still haunted her.
However, she was supposed to be the pony who represented the Element of Generosity. It was selfish not to give Twilight a gown that everypony would be impressed with. She was never going to make the Princess Dress again - the very thought of it nearly made her faint - but she knew that she could take inspiration from it.
Twilight had left the scroll behind. She reluctantly looked at the photograph of the ghastly dress from different angles, working out what she half-liked about it and what she could improve on. A new idea formed in her head. She sketched in her portfolio and created a new design, in between talking to ponies in the store and adding adjustments to other outfits. It took a few hours to get it just right, so that she knew what she had to do if Twilight liked the design. Then she took a picture of it and used magic to send the photograph to Twilight, captioned Evening Star gown. Your opinion?
The reply came almost instantly.
Dear Rarity,
I really shouldn't have asked you to make another Princess Dress. It shows that you never stop learning about friendship, even if you have learned it all before. You make mistakes every once in a while. That dress design you showed me really is beautiful, though. If you're not too upset, could you make it for me?
Your friend always,
Princess Twilight Sparkle.
Rarity chuckled. Twilight would never outgrow her habit of writing formal letters even to friends. At least she liked the design.
She got straight to work the moment the store closed, asking Sapphire and Diamond to fetch different fabrics for her. Twilight had given her a large mannequin that had the same bust, waist and height as her about three years earlier. It had been used quite a lot, but still looked almost new.
"What dress are you making for Princess Twilight?" Diamond asked. The young silver colt kept jumping into to see if he could land on the mannequin's back, not thinking even once to use his magic to help him.
"It's a dress for the Grand Galloping Gala. It's going to be different hues of purple and pink, with a bit of turquoise mixed in. I'm not sure about the fabric yet. Lace definitely will not do, and I probably won't use silk or velvet, but I don't know what else to use."
"Will it have any crystals on it?" a curious Sapphire inquired.
"Well, in my opinion, it would look better without. Have a look at the picture, it's on the desk over there."
She scrunched up her pale blue face and used magic to carry the portfolio to where she was stood.
"Ooh. This is really nice!" she squealed. She coughed and looked away. "I mean, it's ok, but I'd change some of the different coloured highlights. You can barely see the green because there's so much pink everywhere. I quite like the pink, but I still think it's not quite right."
"Tell me what you would do, darling. I might have gone wrong somewhere."
She looked at the drawing. "Well, from what I can see, it looks like there are two layers. One on top, which is mostly purple and slightly see-through, which is where the pink and green layer shows through."
"Hmm?"
"You need a clear dividing line between the pink and green. It's like a dress underneath the dress that you can only just see, but it's still a dress and it needs to look good. If there's pink and green everywhere then it won't look as good. What about having mostly pink, but changing to green at the bottom of the skirt?"
"Oh, yes. You might get your cutie mark in designing outfits yet." Rarity rubbed her daughter's head with a hoof. "Clever filly. I have another idea for the second layer. I'll do as you said, but since this dress will be called the Evening Star gown, I think it needs more than one star on it."
Twilight did not see the dress until the night of the Grand Galloping Gala. She felt some guilt after asking Rarity to essentially relive her worst memories, and had insisted that the seamstress take the money for creating the outfit. The moment she saw it, she knew that all was forgiven, however.
A chest piece, similar to the one she wore on a daily basis but purple instead of gold and encrusted with a pink gemstone carved into the shape of a six pointed star. A gown that changed from deep violet to royal purple gradually, with hghlights of bright pink at the top of the skirt and teal at the bottom. Also, barely visible, there was a soft pink six pointed star on each side of the dress.
"Oh, Rarity, stop making me feel so bad about myself, you didn't have to do-"
"Twilight. We're friends. Of course I had to do this and make a wonderful gown for you. Let's forget the Princess Dress ever existed. If the journalists ask about it, throw them in Tartarus."
They laughed, then walked into the throne room together to enjoy the party.
Author's Note
This idea popped into my head randomly and I couldn't un-think it. had to write something or I would have gone crazy. I also drew this:
Have a wonderful Hearth's Warming, everypony. 
