We already tweeted about this but here's a little more detail. The new online Pokemon merchandise store, PokemonCenter.com is now open, and to celebrate they are giving away the special PokeBall-pattern Vivillon!
Since PokemonCenter.com is for America only, so is this event. It runs from August 7th to August 12th so be sure to get it while you can! It's available via Mystery Gift in the main menu, then select Receive via Internet
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However, the event has also been announced for Europe and Australia, running from August 29th to September 8th. As above it's a short distribution so mark your calendar so you don't forget!
Finally, another event is due to start shortly in America - from August 13th either Heracross (in X) or Pinsir (in Y) are available via Mystery Gift over Wi-Fi. Heracross has the moves Bullet Seed, Pin Missile, Close Combat and Megahorn. Pinsir has the moves X-Scissor, Earthquake, Stone Edge and Return.
In addition, from August 18th participating GameStop stores will provide the appropriate Mega Stones for them to hold and Mega-Evolve in battle! This requires a serial code from GameStop so make sure to get there while stocks last.