Several years had passed since the end of the legend of the "mare in the moon". The time and tale of Nightmare Moon had long been forgotten except by Princess Luna and Princess Celestia themselves, and Celestia had made it clear Luna was forgiven and very loved. Though Luna struggled a lot to forgive herself over her past, badly enough to the point she psychologically tormented herself for a period of time, the help of her sister and her friends who embodied the Elements of Harmony pushed her through it. In the end, it took one powerful heart-to-heart talk between Celestia and Luna, and a lot of tears shed while in her sister's comfort, for her to finally let it go. Afterwards, they never spoke of Nightmare Moon again and over the incoming months, she truly began to feel like Nightmare Moon had never existed. That same year, Celestia secretly created what would be the annual Winter Moon celebration in honor of Luna and the moon, and for Luna's birthday that year. She pretended it was an ordinary celebration, only announcing it on the day of the event, in front of the whole public with Luna front and center. Though initially shocked, Luna was speechless with gratitude and while she had already moved on by this point, it truly hammered in that she had more than amended for her mistakes and Nightmare Moon would forever remain nothing more than an extremely distant memory.
As Luna stood today, she was a fully grown mare, as tall as her sister, who happily ruled Equestria by her side as her equal as the Princess of the Night, and they shared their citizens' love for them. They also ensured they took time for each other as sisters, no matter how crammed their schedules became, and it proved to be worth it, allowing their relationship to stay closer and more open than they remembered. Sure, they still had their petty bickers, but less than a day and an apology later got them over it.
Weeks before one birthday of Celestia's, Luna worked with Twilight Sparkle, her first friend and who she knew Celestia referred to as her most faithful student, to repair the shattered magic mirror and allow Celestia to be with the love of her life again. It'd been difficult, but they finished just a day before Celestia's birthday and presented it to her the next morning as a gift as soon as she woke up. Celestia had been too joyous for words and, though initially nervous, headed through to get back her first love, the mirror world's Sombra, for whom her feelings had never faded. To neither Luna or Twilight's surprise, they returned, their love for each other still strong, and were married by Luna with a wedding ceremony shortly after. To keep things in balance, their world's reformed Sombra traveled to the mirror world to reside there. Luna got along well with her new brother-in-law, who also thanked her and Twilight Sparkle profusely after Celestia explained they'd been the ones who made their lovely reunion possible.
In the Crystal Empire, Princess Cadence and Prince Shining Armor helped their kingdom flourish over the years under their rule. Their daughter, Flurry Heart, was now a teenage alicorn mare. Due to not wanting to receive so much unwanted attention in public, which was inevitable because of her status as the daughter of her kingdom's rulers and an alicorn, she was homeschooled through private tutors. To help her learn about and control her powerful magic, she was taught by Sunburst and Starlight Glimmer, and it was a responsibility the old friends took on with great pride and gratitude.
Twilight Sparkle, now very experienced in her duties as the Princess of Friendship, worked hard to spread its magic and bring many ponies together through the help of the cutie mark map and her School of Friendship. She and her friends had less time for each other over the years, as they'd gone down their own separate paths, but they kept their friendship intact through letters. Much technology had been developed over the past years, but they stuck to letters for the nostalgic feeling, and they occasionally still got together for an adventure or two. Were Twilight ever truly in need, she could still count on her friends to be there for her, and she gave them the same regard. Spike remained her loyal friend and assistant, and she didn't miss a chance to remind him he was family to her either.
Apple Bloom, Sweetie Belle, and Scootaloo, still known as the Cutie Mark Crusaders, had grown up to travel Equestria in their mission to help others discover their cutie marks. However, they too followed paths that occasionally separated them from each other. Apple Bloom returned home periodically to assist on her family's farm, Sweetie Belle occasionally performed songs at charity events, and Scootaloo competed with Rainbow Dash in races in the latter's hometown of Cloudsdale. The CMC always reunited to continue their mission, declaring they'd keep traveling until they no longer could.
Everypony was living a busy life, but a happy one.