Last month, Mew became your free Pokemon of the month to celebrate Pokemon's 20th anniversary. It was limited to Game stores in the UK and Gamestop in the U.S though.
This month Nintendo's doing things a bit differently has anyone can get this month's free Pokemon: Celebi. It's available if you have a copy of Pokemon X or Y or Pokemon Alpha Sapphire or Omega Ruby.
To do so just select "Receive Gift" and then select "from the Internet". You will then find it in the nearest Pokemon center.
Or you can also view those instructions here.