The official Pokémon newsletter is currently sending out emails containing codes to download the legendary birds Articuno, Zapdos, and Moltres. Titled “Get the Mythical Pokémon Darkrai in May!”, the email will have a code for each bird in the middle of the newsletter.
Once you receive your codes you can then activate them in a copy of Pokémon X & Y or Pokémon Omega Ruby & Alpha Sapphire. Simply go to the Mystery Gift option and select the prompt “Get With Code”.
For those looking to join the newsletter, you can sign up for the service here.