@Red-Supernova  
Problem is Celestia has this huge lead via a 1000 years.
 
Luna is known due to she was ruling.  
But now that she isn’t anymore, she’s not going to be ultimately remembered much due to she didn’t rule anywhere near as long as Celestia did. Luna lost a 1000 years of ruling and thus didn’t have much true impact on Equestia as whole compared to Celestia.  
Like I said, Celestia is going to be remembered WAY MORE than Luna due to this huge problem there.

@nightshroud96  
I don’t think ponies are going to be forgetting Luna. She’s been well-known ever since her return.

While its heartwarming Twilight renamed the celebration to celebrate both sisters.  
Problem is it doesn’t fix the issue of how much love day has gotten over the night.  
A 1000 years of how much love Celestia’s day has gotten, that is still a problem.  
Along with how Luna didn’t get to rule for much long and thus not have much impact on Equestria compared to Celestia.  
So even if this festival includes Luna, the ponies are obviously going to remember Celestia much MUCH more than Luna.  
Especially with how apparently history forgotten about Luna for a 1000 years after she got sent to the moon.

This is another episode where I still don’t know how to feel about. I would have liked it if the Mane Six knew that all of these problems going off weren’t just a coincidence.
 
@Background Pony #3FF5  
YES. It would have given the later half of Season 9 some more needed hype.

I genuinely laughed when Cozy smashed the lock with a book that just so happened to have a key symbol on the cover. That was just plain genius.

The episode that made us realize the villains were more interesting. They should have been in more episodes.

I thought the villains worked well together again in this one.