Image imported from derpibooru 
Original Description:
 
While Celestia got older, her mane once more began to shift- something that hasn’t happened since she grew into her Alicorn stature for the first time, much like Luna was prior to retirement. The pink in her mane sunk to the bottom and the other colors changed into much warmer tones. It was then she realized that, even with a lifespan as long as hers, she is not immortal. She chose not to inform any of the other Alicorns however… They don’t need to know just yet.  
Meanwhile, Luna began to wonder- what of the Dream Realm? She doesn’t know of anypony else that can travel there- Well, there was Twilight, but she has enough on her plate with Celestia’s duties. After much consideration, Luna chose to come out of retirement and return to the throne- not as the Princess of the Night, no, Twilight has long taken over both Day and Night, but rather as the Princess of Dreams. Besides, she hadn’t ruled as long as Celestia did, and she figured she may as well make sure the two of them are even, Sibling Rivalry and all that.