Starswirl the Bearded developed a time-travel spell. A pony could use it - but only once, and only be in the past for about a minute.
One chance. One minute. That's all Princess Celestia has to save her sister from the Nightmare.
A special thanks to daOtterGuy for answering when I said "Name me four stars in the Equestrian sky" and unwittingly helping me with this.
Featured 7/27/2023 - 7/30/2023!
Now with a Russian translation by PlaguedGuard.