Alongside the new update for Pokémon Bank to make the service work with Pokémon Sun & Moon – reported earlier today – there is also a special item available for users.
As a method of celebration of the update, all Pokémon Bank subscribers can grab themselves a Mewnium Z, a Z-Crystal for Mew and Mew alone. Upon receiving the gift through booting up Pokémon Bank, you will have a new Wonder Card appear in your Pokémon Sun or Moon game.
From here, you can use Mystery Gift to get the item. The item is accessible in this way until the cut-off date of October 2nd 2017, and only one Mewnium Z can be obtained however many copies of Pokémon Sun & Moon you connect to Pokémon Bank.