The Mystery Gift feature from Gold and Silver makes a return in the form of a new feature, Mystery Event, and like in those games it must be unlocked in game by talking to the right NPC. However this time you need to know the correct phrase to insert during an interview. This time it was just used for events and E-Reader compatibility. You must have defeated your father Norman at Petalburg Gym to access it.
First go to Petalburg City Pokemon Center and find the person by the PC. When asked, insert the phrase "Mystery Event Is Exciting" and you'll be informed you can now access Mystery Event. To do so however you must save the game at least once after entering that secret message and load the game from the start. On the menu after the title screen you will find you can perform Mystery Event now!