To celebrate the continued ‘Year of Pokemon’ commemorating the series’ 20th Anniversary, the Pokemon Company will be distributing multiple legendary creatures in the months to come. Mew was recently given out at GameStop stores, and it has been revealed that Celebi will be given out this month.
— Pokémon (@Pokemon) March 1, 2016
Players can connect to the Nintendo Network on their 3DS with either X and Y or Omega Ruby and Alpha Sapphire versions of the game to claim a level 100 Pokemon for free. The dates Celebi runs are March 1-24, 2016.
For instructions on how to connect and download this Pokemon, check the official website.