Help How do you catch darkrai in Pokemon diamond?

I tried to go to the island but the door is lock? How to you unlock the door? My friend told me that I need to go to an pokemon event but I can not find one? If anyone have a darkrai ( not using AR or cheat things) and want to trade, I will trade strong pokemons and any legendaries like jirachi, celebi, or... show more I tried to go to the island but the door is lock?
How to you unlock the door?
My friend told me that I need to go to an pokemon event but I can not find one? If anyone have a darkrai ( not using AR or cheat things) and want to trade, I will trade strong pokemons and any legendaries like jirachi, celebi, or manaphy.......and a master ball.

Please help me I really want a darkia!

Thank You!

My FC is Binh P.
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