What do you do when you're a princess who doesn't understand a joke about you made by a certain chaos master? You give up and finally ask him over to explain things once and for all. So Celestia wants to know--why would Discord call her K.K.?
Luna rules the night. So when Celestia returns to the castle in a bad mood after Twilight's failed dinner party with Starlight, the night princess knows it is her duty to set her sister's mind at ease (as well as the minds of a few other friends).
It's Grand Galloping Gala time! Discord, now fully reformed, is surprised when Princess Celestia both invites him to the party and makes a special request of him as her friend. Celestiacord. Tons of Discord!
"Too!" Get it? Because others are also the same, but it also sounds like the number... Anyway, after several months, OOREV is still going strong (more or less). Which means there's only one thing to do--add more members! Discord insists!