The official Pok√©mon Black and White website reports that people who purchase either Pok√©mon Black or Pok√©mon White between March 6 and April 10, will get a head start on their mission to catch ‘em all. They will be able to use the Nintendo Wi-Fi connection to receive a special item, the Liberty Pass, which will give them a chance to catch Victini.
The Liberty Pass is an item you can use in your game to hop aboard a ship and sail to a location where you can catch the Mythical Pok√©mon Victini! Victini is a Fire- and Psychic-type Pok√©mon‚ÄĒthe first of its kind. And Victini has the Victory Star Ability, which raises the accuracy of all your Pok√©mon in battle, so it’s the perfect Pok√©mon for the Double Battles and Triple Battles you’ll play!
Using the Liberty Pass is the only way you can receive Victini in your game‚ÄĒyou won’t encounter this awesome Pok√©mon in the wild. The chance to download the Liberty Pass won’t be around long‚ÄĒyou can receive it via the Nintendo¬ģ Wi-Fi Connection service from March 6th through April 10th. Don’t miss this chance‚ÄĒthis is the only way you can get the Liberty Pass.
Also, you can download the Liberty Pass as soon as you begin playing your new game, but you’ll need to reach a certain point in the game before you can use it to catch Victini.
Source: Pokemon Black and White Website.