This was used as an image for Neighsayer's 'At Least 1000 Years' on youtube, so for me It's Celestia the day after banishing Nightmare Moon. ---
It's the start of a new day and Celestia is, with great pain in her heart, for the first time ever in her life setting the moon and raising the sun, and for the first time ever also truly alone. She realizes she's now on her own not only to rule the Equestria, but also alone to live her immortal life. To live it in this beautiful world, while Luna, turned monster, but nonetheless her sister, is damned to existence on a vast cold barren of the moon. She is contemplating about what she did just a few hours ago. If she did the right thing, if she even had right to do it, if she could have done anything else to stop Nightmare Moon, whether there was any other way... And it dawns to her, as the new day is slowly waking, that her, despite staying in world filled with life, is in the exact same situation as Luna.
For who knows how long will it be untill they'll see each other again? It seems like eternity to her. The sun will rise and set thousand times, the world will change hundreds of times, generations will come and go, but loneliness inside her will never cease until she will see her sister again one day.
But watching sun rays brighten the sky and coat the very earth in their magnificent gold hue, deep in her heart Celestia finds resolve, finds hope.
For to her, although she is immortal, time has a meaning.
Beacuse time is not only her or Luna. Time is the whole world. Time is the lives of those ponies who live here, time is the lives of their descendants and descendats of those. Every single one unique in all of creation. How she could let this slip away? She will be here for them, she must. It's bigger than her. And one day, after aeon which would seem to her as just a blink of an eye, one day which will come sooner than she would ever expect, of that she is now sure, she will see her sister again.
.......... (now I imagine her taking off and flying off in some unspecified direction) Ok, I got a little carried away there with those long sentences and whatnot (and then there's that thing that English isn't my first language), so I hope it's readable.
Please, take it as my tribute to your work, Saros.