How do you catch a legendary in Pokemon diamond?

You can't catch a legendary Pokémon in Pokémon Diamond directly. Pokémon Diamond does not feature any legendary Pokémon that can be caught in the main game.

However, you can obtain some legendary Pokémon through:

* Trading: You can trade with other players who own Pokémon Pearl or Platinum to acquire legendaries that are exclusive to those games.

* Events: Nintendo sometimes hosts events that allow players to obtain legendary Pokémon by connecting their game to the internet. These events are usually time-limited.

To find out if there are any current events or trading opportunities, check the official Pokémon website or online forums dedicated to Pokémon trading.