"These six Shiny Pokémon that are or evolve into Dragon-type Pokémon were distributed over Wi-Fi. There was a total of six different time periods, allowing players six different opportunities to obtain all six of them."
Source: bulbapedia
We just don't know for sure if each pokémon has his own time period to be obtained, or if they are distributed randomly.
It doesn't really matter, this seems to be the best pokémon event ever!