Serebii is currently reporting that the GTS has finally hit 100 million trades, and the dates for the Fancy Pattern Vivillon have been announced. The download is supposed to last from July 7th - July 31st. As of 8:30 am EST, there is currently nothing about it posted on the Pokémon Global Link site or Pokémon.com. This new pattern of Vivillon was officially unveiled in May, and is meant to commemorate the 100 million trades that have been done over the GTS since pokémon X and Y came out. As such, the OT of the Vivillon is GTS and has the ID number 00108. It is lv 12 andiIt also knows the event exclusive move, Hold Hands.
According to Serebii, Hold Hands does not do damage but rather "Ally pokémon hold hands. This makes them very happy." Most likely this means this is an effect boosting move like Happy Hour, the only difference is Happy Hour boosts the amount of money won in battles whereas it seems that Hold Hands will increase the happiness of the pokémon in battle. If so, it will help people evolve pokémon that are based on happiness such as Eevee to Espeon. In addition to Hold Hands, it also knows Gust, Struggle Bug, and Light Screen.
To receive this Mystery Gift, you have to be in range of a wifi hotspot.