If you just picked up Pokemon Sun and Moon and are eagerly powering through the land of Alola, you’ll want to make a note on your calendar for December 5.
Starting then, if you’ve completed your journey therough Alola you’ll be able to pick up the mythical Pokemon Magearna in-game.
When you finish the main story you can head to Pokemon.com and unlock a QR code with the QR scanning feature on Pokemon Sun and Moon, where you’ll be rewarded with the Magearna.