All episode information can be found after the break! I don't think Synopsis are really spoilers, but you have been warned I suppose.
Season Two Episode 4: Luna Eclipsed
Air Date: October 22, 2011
Synopsis: It's Nightmare Night and the macabre celebrations are in full swing when Luna herself shows up in Ponyville, determined to change her frightening public image.
So our first encounter with Luna is a Halloween episode!
This also sort of confirms the leaked images from yesterday.