The official site has announced that the Oak's Letter item is to be distributed over Mystery Gift over WiFi for Pokémon Platinum in America from Monday, September 28th to November 8th. This item gives you access to an extention of Route 224 that leads up to a wild Shaymin. This Shaymin, like previous event Shaymin, will unlock the Gracidea in your games to allow you to change your Shaymin into its Sky Forme.
