Lol, I never realized how funny it is that saying "Good morning" to Luna is her equivalent of saying "Good night" to someone who's diurnal (sleeps during night). It just sounds so hilarious in this quote from a fanfic I'm reading:
"'We think we are done for the evening, we are tired!' Luna suddenly rose with enough force to knock her chair over, clearing her throat. Blueblood turned curiously, looking back and forth between the royal sisters. 'Good morning, sister! Nephew!' she strode away quickly.
'Good morning, Luna!' Celestia called weakly, shooing off a servant that was trying to help pat tea out of her fur. She coughed a little, trying to recover her dignity. 'Well! I must go and finish coaxing the sun skyward, good day nephew!' she quickly strode out of the room the other way"