https://www.polygon.com/e3/2018/6/12/17455954/splatoon-2-octo-expansion-release-date-dlc

The first paid expansion for Splatoon 2 is coming Wednesday, June 13, Nintendo announced today at E3 2018. Splatoon 2 producer Hisashi Nogami revealed the Octo Expansion’s release date today at the end of the Splatoon 2 World Championships.

Splatoon 2: Octo Expansion is a new single-player adventure that packs in more than 80 missions. Players take on the role of Agent 8, a new character who fights her way from an underground facility to the surface of Inkopolis. Players can unlock playable Octolings and in-game gear by completing the Octo Expansion DLC.