https://www.pokemon.com/us/pokemon-news/a-new-movie-and-a-new-pikachu-for-you/
Starting September 19, check back here on Pokemon.com to discover a code that allows you to receive a Pikachu wearing one of six different hats in your game. Each hat is based on the hat Ash wore in the various regions he and Pikachu explored. Not only will each Pikachu sport a different hat, but they will also come equipped with a unique set of moves that reflect those that Pikachu would often use in the region that corresponds with the hat it wears.
PLEASE NOTE: You will only be able to use this code once! The date you choose to enter the code will determine which hat and move set your special Pikachu will have. This means you're going to have to be very careful when deciding on which Pikachu you wish to add to your party. Study the schedule below, and get ready to choose your Pikachu!
9/19/17 – 9/25/17: Original hat (Kanto and Johto regions)
9/26/17 – 10/2/17: Hoenn region hat
10/3/17 – 10/9/17: Sinnoh region hat
10/10/17 – 10/16/17: Unova region hat
10/17/17 – 10/23/17: Kalos region hat
10/24/17 – 10/30/17: Alola region hat