Princesss celestia has always brought light, and luna, dark. after the summer sun celebration, luna has been bringing a professional dark to equestria, as it was a thousand years ago.
Everypony has gotten used to this darker dark. well, everypony except fluttershy.
Celestias bringing forth of the night was just to the standard dark for fluttershy to rest peacefully, but now, fluttershy is not so well aquainted with the darkness of the night.