This week, through this coming Sunday the 23rd, the Pokemon to be downloaded is Entei.
You need these Pokemon for Pokemon Black and White. When you transfer them to Pokemon Black or White you will unlock the ability to catch the new Pokemon Zoroark. Zoroark will otherwise not be obtainable in the game.
It sounds like you will need all three for the unlock, so head to GameStop before this coming Monday for the download.
The final download event will follow in two week.
Suicune – January 31st through February 6th
That fill then be followed in some areas by the Pokemon tour.
These Pokemon will be special. Not only will they be special “shiny” versions of the three legendary Pokemon, but when you transfer them to Pokemon Black or White it will unlock the ability to catch the new Pokemon Zoroark. Zoroark will otherwise not be obtainable in the game.
This is an in-store only event. You must got to a US GameStop store during the dates listed to be able to download these Pokemon.
However, since it is an in-store event, you will be able to download these Pokemon to Diamond, Pearl, Platinum, HeartGold and SoulSilver.
Details about the event and how to participate are available at Pokemon.com.