The Pokemon Company announced that a Vivillon with a Fancy Pattern is now available via Wi-Fi in Pokemon X/Y. This Vivillon is being distributed to commemorate 100,000,000 Pokemon traded via the Global Trading Station. The Fancy Pattern does not normally appear in-game, so it’s only available through this event.
To get this Vivillon, get your 3DS online, select Mystery Gift on the main menu, then Receive Gift, then Get Via Internet, then finally pick it up at a Poke Mart. It comes at level 12, male, and with the moveset Light Screen, Gust, Struggle Bug, and Hold Hands. The Hold Hands move is exclusive to this event Pokemon.
This Vivillon will only be available until July 31st, so you have about three weeks to grab it.