At the beginning of Pokémon Brilliant Diamond and Shining Pearl, you won’t have access to the Mystery Gift feature. Once unlocked, Mystery Gift allows you to receive Pokémon and items when specific events are active. This feature will be automatically unlocked after beating Gym Leader Maylene in Veilstone City, but there is a way to unlock it earlier.
Unlocking Mystery Gift as fast as possible
When you first visit Jubilife City, a clown NPC will be blocking the door to the Jubilife TV building. You’ll have to wait a bit longer to unlock the Mystery Gift feature, so continue with the story and where the game prompts you to go.
After defeating Gym Leader Roark, returning to Jubilife City and defeating the Team Galactic grunts, you can visit the Jubilife TV building. On the third floor, an NPC will ask you what your favorite TV show is. You need to answer “EVERYONE HAPPY WI-FI CONNECTION” and this will unlock the Mystery Gift feature well ahead of the automatic unlock.