I’d hate to make you read through all of this and keep your hopes up for a definitive answer the whole time, so I’m just going to spoil it all now and say that there actually is no meaning behind the “-P” in “Luna-P.” But wait! Before you move on, you should at least stay for the back story behind this mysterious appended letter, because there’s actually a rather interesting story behind it!
I don’t think I need to really go out and say that dates are a little strange in the Sailor Moon universe, especially when it comes to calendars, dates, and ages. The series, in the manga at least, does a surprisingly good job compared to other long-running series, but that’s a story for another time. But one thing that always stood out to me is: why, exactly, do ChibiUsa and Usagi have the same birthday?