The official Pokémon website has announced that Meloetta will be distributed to players of Pokémon Ultra Sun, Pokémon Ultra Moon, Pokémon Sun, or Pokémon Moon who are attending the Pokémon World Championships 2018 in Nashville, Tennessee, United States on August 24th, 2018 through August 26th, 2018.

Get the Melody Pokémon Meloetta at Worlds

The Mythical Pokémon Meloetta will raise its voice in Nashville this August at the Pokémon World Championships! Nashville is famous for the many talented musicians whose names were made there, making it the perfect place for Meloetta to take the stage. All Pokémon World Championship attendees can get a performance-ready Meloetta holding a Metronome and knowing a full set of musical moves. This is a momentous event, as Meloetta is the first Mythical Pokémon to be given out as a Pokémon World Championship event.

If you're attending the Pokémon World Championships on August 24–26 and want to add this special Meloetta to your Pokémon collection, remember to bring your system in the Nintendo 3DS family and your copy of Pokémon Ultra Sun, Pokémon Ultra Moon, Pokémon Sun, or Pokémon Moon. You can get only one Meloetta per game. Meloetta will be available outside of the main event hall, so you don't need a Worlds entry badge to receive the Pokémon.

Look for signs at the Pokémon World Championships for distribution times and instructions on how to get Meloetta. Meloetta

Level: 50
Ability: Serene Grace
Held Item: Metronome
Moves: Celebrate, Relic Song, Round, Sing

Source: https://www.pokemon.com/us/play-pokemon/worlds/2018/pokemon-distribution/