Starting July 31, North America players of Pokemon HeartGold and SoulSilver have 28 days to download the Enigma Stone, the key to unlocking Legendary Pokemon Latias and Latios.
Legendary Pokemon Latias can be captured in Pokemon HeartGold. Her brother Latios can be captured in Pokemon SoulSilver. Both are exclusive to those games, unless the player has access to an Enigma Stone.
How does one get an Enigma stone?
From July 31 through August 27, The Pokemon Company and Nintendo are making the Enigma Stone available to Pokemon fans in North America. Players with wireless access will be able to download the special item, which eventually leads to a chance at catching Latios in HeartGold and Latias in SoulSilver.
In order to use the Enigma Stone, players will need to have beaten the Elite Four, claimed the National Pokedex, and have two or fewer Wonder Cards in their possession. Bulbapedia has a nifty little summary of how the deal goes down.
Just make sure you go online and claim your Enigma Stone before August 27. Then you can start worrying about how to use it to get dates.