...about five minutes.
I guess this comes to support the 'hasty decision' hypothesis on why Celestia sent her to the moon, and turns into AU's all stories about a bloody war happening between them. Heck, NMM didn't even do that much damage to the castle, so why did Celestia leave Everfree castle and move to Canterlot?
Anything else to add?

2229185
Considering how short it lasted there is no way it would have left enough scars on the ponies for them to actually create a day about how much of a tyrant she was.

2229185

I don't really know if you need to black out text when you've already announced spoilers, but

Celestia probably moved to Canterlot because having to spend all day and all night in the place where she was forced to make the hardest and most regrettable decision of her life would probably make her into an emotional wreck within the space of 1000 years.

2229185 Hence as to why I prefer the fanfics.

2229231 long live head cannons

2229268

Cannons, you say?

2229286 yes

2229286 That looks more like a face cannon, but it still works.

2229204
At that, I would imagine that the holiday pre-dated her going crazy (being called something like "Autumn's Moon Celebration"), but then rapidly evolved into a mockery of her.

2229268
yes.

2229371

Maybe Celestia was disappointed by how poorly her sister's rebellion went, so she made up a grandiose story about it to preserve her reputation.

I am mostly joking, but come to think about it, it might have been necessary. The Elements appear to have been dormant between the banishment of NMM and S1E01, so Celestia would need to talk big in order to make sure nobody tested Equestria's defensive abilities.

The story 'Oh, she said some evil stuff, then she chased me around, and then I zapped her with the Elements, the end', doesn't tend to intimidate.

I'll grant it was a bit short, but it was always my headcanon that the Nightmare Moon incident was fairly brief and mostly just involved the principal players. This is even shorter than that, though; how did anyone get a good enough look at Nightmare to build a statue? Were there even other witnesses? Is this why everyone is always so terrified in Celestia's presence? If she knocked off her own sister one night when no one was around, well, what is she going to do to you?

Heck, I can imagine the next shift of palace staff coming in, putting their reports in Luna's in box, and not having the vaguest clue that she's not working there any more. They'd be all, "Oh crap! Seriously!? She seemed fine yesterday! What the hay happened?"

2229185 Stories about long, drawn out wars between Celestia and Luna are and always have been stupid! Yes, I'm talking to you Winningverse! (i still love you).
2229205 I can see her moving to the mines of Canterlot in only a few years and leave the Elements in her own castle to be safe and forgotten. She might have been sweet and lovely but to protect Equestria she turned into a Princess much more like Majesty in G1.

... Whelp, I suppose it is time to force everyone to adopt the new canon.

ADOPT THE NEW STANDARD OR BE SHOT!

After all, we cannot have worthless traitors to the purity of the show litter this fandom, correct?
Purge the scum from the site en mass. Those who do not conform must be deleted and banned.

You should've dressed as a bannana, Celestia! I'm Nightmare Moon, Nightmare Mooooon!

(That said, I was dissapointed that Luna was always smaller. That and her missing ear.)

Well considering that Luna was shown to be malevolent before becoming Nightmare Moon, and that Celestia wasn't completely shocked at what became of her, I'd say Luna had made her stand for monarchy significantly before she was Nightmare Moon. What Twilight saw was just the climax of it all

2229185 I personally endorse a differing MLP universe when writing that follows cannon to a degree but is far more realistic and gritty in its approach, such as having the expedition into the Everfree taking weeks where each of the barres show their traits in more subtle ways.

As for your question it was likely because it was a place full of painful memories for Celestia.

On aside note it would have been extremely interesting to see the battle against Nightmare Moon split into seven parts allowing each character their own episode.

Oh! And this also confirms my personal head cannon the Celestia and Luna were the rulers of Equestria before Discord came into the picture.