Depending on what version of the recent Pokémon you played, either Omega Ruby or Alpha Sapphire, you got either a Latios or Latias during the storyline. Now's your chance to get the other one.
By booting up Omega Ruby or Alpha Sapphire today and going to Mystery Gift, Receive Gift, and then Get With Code and entering 2015LATIOSLATIAS (or POKEMON380381 if you’re in Europe) you will get a special ticket. You can pick up the ticket at any Pokémon Center, and then take it to your father, the gym leader in Petalburg Gym, and he will place you on a ferry to go to an island where you can battle and capture whichever Legendary Pokémon you don't already have.
If you're playing Omega Ruby, it will be Latias. If you're playing Alpha Sapphire, it will be Latios.
For our review of Pokémon Omega Ruby and Alpha Sapphire, head here.