https://www.gamespot.com/articles/free-legendary-pokemon-for-ultra-sun-and-moon-avai/1100-6456411/

The Pokemon Company has a ton of giveaways lined up for Pokemon Ultra Sun and Ultra Moon this year as part of its Year of Legendary Pokemon, which will give players a chance to receive a free Legendary each month in 2018. That starts this month with the first pair of free Legendaries, Dialga and Palkia.

From now until February 28, players can pick up a download code for the Space and Time Pokemon from a participating GameStop location in the US; UK residents will be able to obtain a code from Game stores during the same time frame. The code can be redeemed in either Ultra Sun, Ultra Moon, or the original Sun and Moon versions, but the Pokemon you receive will depend will depend on which game you have. You can see which Pokemon you can get and their moveset below: