CLAMP day event announcement
The fun part is, of course, the creatures showing up will also be separated into sun & moon types, and only the opposing skills will be strong against them. So if you're a sun type facing a sun spirit, you're gonna need the help of a fellow moon type, or perhaps several other sun ones to make up for the disadvantage.
Powers will fit the characters, too, and not just regarding which of the two they get. If your character is already a fighter in canon - take Arthur Pendragon, who would be a sun type - he'd have a sun-powered Excalibur and he could slash sun magic at enemies in a very JRPG fashion. But someone like Sakura, meanwhile, could have mostly defensive skills and produce very bright, sunny shields to protect others while they fight.
There will be more details - on the monsters they're fighting, for example - once the log goes up on the 15th. The event is set to, tentatively, last around a week, and end with an appropriately themed dance that gives closure to the days of fighting, by uniting all the suns and moons together.