According to the German magazine Gamemaster, Member’s Card, the item required to trigger the Darkrai event, will be distributed via Wi-Fi across Europe starting August 3rd.
Once picked up at any Pokémart, go to the Canalave City and you enter the previously locked Inn. Once there, you fall asleep and wake up on New Moon Island where Darkrai lies, at level 50. Once captured, you will hear a deep voice explaining why Darkrai was placed on that island, probably the voice of Arceus.
Source: Filb.de