The Pokemon Company has been distributing one Mythical Pokemon a month to celebrate the Pokemon franchise's 20th anniversary. Most of the giveaways use the "Mystery Gift" function in Pokemon X, Pokemon Y, Pokemon Omega Ruby, and Pokemon Alpha Sapphire, but May's giveaway of Darkrai also required a serial code distributed by GameStop and other large game store chains around the world. Unfortunately, there's only a limited supply of codes at each store, which means that some players could miss out if they don't get to a store in time.
Luckily, Nintendo is offering players a second chance to get Darkrai via their monthly Trainer's Club newsletter. Players who get a newsletter will receive the code in September's newsletter, although it will only work if players didn't receive and use a Darkrai serial code back in May.
If you want to get the Darkrai code, you'll need to sign up for the Trainer's Club newsletter by August 24th. You'll need to first sign up for a Trainer's Club account at Pokemon.com and then make sure you've agreed to receive emails from the Pokemon Company by going to the "Edit Profile" page. You'll then receive an email to opt into Trainer's Club newsletters, with a link to click on to proceed. If you already have a Pokemon Trainer's Club account and don't receive the emails, check your junk mail for the opt in email.