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.
You just download it from Mystery Gift.
Source: Front Page