Once upon a time, in the magical land of Equestria, two Princesses were born. Not at the same time, though you could be forgiven for mixing up the dates, considering they were over a millennium ago. These two sisters would grow up to be the legendary Celestia and Luna, nigh-immortal wards of Sun and Moon. Of course, once you clear up the anachronisms, the story is a bit less glamorous.
For one thing, as magical as it is and was, the Equestria of the time was better described as “chaotic”. Discord, a creature of no small infamy, was using his exceptional power to turn the nation into his own personal playground. For the ponies living there at the time, the only relief was that their overlord was very careful not to break his toys. Or at least, not all of them.
Additionally, calling these two fillies “Princesses” is somewhat jumping the gun. Not only were they not born to royal parents, and so fell short of the traditional meaning; on top of that, they each lacked two magical aspects that would be the standard for Equestrian Princesses. Which is to say, they were not alicorns as of yet. As it stood, it would be far more accurate to call them entirely regular ponies, at least physically — though most would call this heresy.
Finally, and almost as commonly disputed, is that Celestia and Luna were not called that at birth. Indeed, such grandiose names among the pony populace were quite rare at the time. With some context, the reason for this will become quite clear.