North America - September 28th 2009 to November 8th 2009
PAL - September 28th 2009 to November 8th 2009
Despite the fact that Oak's Letter was programmed into Pokémon Diamond and Pearl by Nintendo, it was never officially distributed at any event.
When Shaymin is successfully captured through this event, it is listed as having a Fateful Encounter even though it was caught in the wild. This is due to the fact that only a Fateful Encounter Shaymin can transform into Sky Forme in Pokémon Platinum version.
When the Oak's Letter event is activated, Marley has different dialogue when the player speaks to her.
Although this event was accessible through cheating in Pokémon Diamond and Pearl, since the first Generation IV games did not apply the Fateful Encounter tag to specific Pokémon, it would never be able to activate Sky Forme in the following game.