From Bulbanews, your community Pokémon newspaper.
Uses a common serial code
Update: The distribution has now been officially announced, confirming that it will end on September 30, 2016.
It cannot be obtained on the same game as the Darkrai from the earlier Pokémon 20th Anniversary distribution, and is identical to it.
As previously announced, a second chance serial code for American region games will be sent out via Pokémon Trainer Club newsletter in September, which will be valid until November 30, 2016. Consequently, it is likely that the PAL region common serial code will also be valid until November 30, 2016.
PAL region games are sold in Europe, Australia, New Zealand, South Africa, and some other countries.