She did the same thing in Lesson Zero, though.

They also imply that Celstia was imprisoned in the sun, by having her appear in a ball of light that appears to descend from the sun as it rises.

@Randell  
Thing is, we don’t know exactly what happened, and it’s probably going to stay that way.

“How was it implied? It took the godlike elements of harmony to banish NMM in the first place, what means does NMM have of banishing Celestia? None.” Your comment isn’t logical. If Celestia needed the Elements of Harmony in addition to her own power to banish Nightmare Moon, then that implies that NIghtmare Moon is more powerful than Celestia (at least, when she doesn’t have the Elements of Harmony). Thus, when NMM showed up, she banished Celestia in the sun or otherwised imprisoned her. The sun princess was released when NMM was defeated.

She more than deserves an episode. She’s been important since episode one.
 
If we don’t get love for Scootaloo and Celestia this season, I will be seriously disappoint.

^Forgot to add  
It’s worse when Celestia just serves to get hit with the Worf Effect.

@itsthinking  
It’s sad when Luna has more character development than Celestia even though she only really had one episode.

That’s my line thinker you bastard
 
but
 
yes

We really need a Celestia episode, she’s less a character at this point and more of a plot device.

How was it implied? It took the godlike elements of harmony to banish NMM in the first place, what means does NMM have of banishing Celestia? None.

It was implied that Celestia was imprisoned by Nightmare. Problem solved.  
Now, the plotholes in A Canterlot Wedding, on the the other hand…

@Impious  
That’s probably the best explanation you can get from the limited info given about the past.

*Celestia’s only friend

Or Celestia was in hiding. Twilight and her friends were the only ones who could stop/save her anyways.
 
And it’s not like she meant to banish her sister… As The Return of Harmony showed, the elements are useless when used by those with broken/corrupted hearts. Luna’s only friend is Luna, right? Her betrayal would have utterly crushed Celestia’s heart. In her desperation to save her sister, she must have forced herself upon the elements… Normally they wouldn’t work, but she made them. But instead of fixing her insane sister, she disappeared. As a result, the elements no longer respond to her. She broke her connection to them.
 
So why participate in a battle you can’t be of use in?

Nah we just got invaded and your sister about got killed

“did I miss anything” -Luna

Yeah, it seemed pretty clear after Celestia’s disappearance at the Summer Sun Celebration that Nightmare Moon had done away with her in some way.

Eh, I personally don’t want to get into the plot inconsistencies in the two parters. (Luna pretty much being shoehorned in the wedding episode just to let other’s know she exists.) I’ll just stick to the nice slice-of-life stories.

@Bobrella  
I take it as she didn’t want to fight, so she just kind of hid to avoid unesscessary bloodshed.

It was all part of her plan: she’d carefully groomed Twilight and the others to be suitable bearers of each of the Elements of Harmony, so that when Nightmare Moon returned, she could be purified rather than banished again. If she hadn’t, her subjects would’ve expected her to do it herself, which she couldn’t–if she could use the Elements to their fullest, she would’ve done it 1000 years ago.

@soundtea  
Nightmare Moon didn’t have the power of the elements. She also didn’t have a) friends, b) the willpower to use the elements, or c) the foresight for when she returned to Equestria that the true element bearers had showed up.

@ROTHY  
Still, she could have at least helped them a bit in the start if her entire kingdom’s on the line.

@ROTHY  
Damn your being a little bitch about a meme lol

this again

@SoundTea  
Because NMM didn’t think of doing that first.