How to get Sylveon in Pokemon X and Y

Pokemon X and Y's only evolution that comes from a previously released Pokemon is Sylveon, the newest Eeveelution. But with Eevee already having a ton of different evolutions that take advantage of nearly every element of the Pokemon games (different stones, different times of day, different moods), the actual means of turning Eevee into Sylveon were unknown. Many speculated it would involve Moon Stones