Celestia raises the sun, and carries it across the sky. Luna does the same with the moon. It's a simple cycle, as old as the defeat of Discord.
It's not entirely accurate.
Though the Sister Princesses do rule the day and night, what of the hours between. Who rules the twilight hours of Dusk and Dawn?
Why, their namesakes, of course.
Where are Dusk and Dawn? Why aren't they know to the world? And why does Dusk refuse to speak of Celestia?