Last month you could get Mew via the Mystery Gift option in Pokémon XY and ORAS. This month until the 24th, you can get Celebi, knowing the moves Confusion, Recover, Heal Bell, and Safeguard. After you’ve gone through Mystery Gift on the main menu, you can retrieve it at a Pokémon Center. As with all the mythical Pokémon in this give-away, it will be level 100 when you receive it.
Starting next month, you’ll be able to download Jirachi. Check the official website for more information and to see the full year’s line up of mythical pokémon.