Totally forgot to post this a few weeks ago, but a new Pokémon event is currently active! Since the GTS in Pokémon X/Y reached 100 million trades, a new Vivillon pattern has been released.
This one is called the Fancy Pattern Vivillon and it comes with an exclusive move, Hold Hands, which increases Pokémon's happiness (but can only be used in double/triple battles).
It is available via Mystery Gift
on the main menu, then Receive Gift
, then Get Via Internet
. The event runs from July 7th to July 31st so make sure to get it while you can!