After a new Pokémon TCG expansion launches, there is a brief waiting period before the new cards can be used in Play! Pokémon competitions, to give players an opportunity to understand and practice with the new cards. Beginning with the Pokémon TCG: Sun & Moon expansion launching February 3, 2017, this waiting period has changed, and several other minor tweaks have been made to the card legality schedule.
Cards from each new expansion will become legal on the third Friday of the same month the expansion was released. So you'll first be able to compete with Sun & Moon cards on February 17, 2017. Additionally, promo cards such as those found in Pokémon TCG Boxes and Collections will be legal on the first or third Friday of the month after the product is released.
You'll always be able to find an updated list of tournament-legal cards on the Play! Pokémon Rules and Resources page.
Have fun building and battling with new decks throughout 2017!