Splatoon 2's new horde-mode-style Salmon Run will only be playable online at certain times. Nintendo confirmed in a Splatoon 2-focused Direct presentation that Salmon Run will always be playable in local multiplayer, but online play will exclusively be available during regularly scheduled events. Here's what the schedule looks like as of now.
Assuming this is the final version, Splatoon 2's first week alone will have multiple 24-hour events (disregard the ongoing event that began back in February, that's probably just for testing). Salmon Run frequency may be adjusted as time goes on; Nintendo says they'll always be more frequent than Splatfests, which occurred about once every month in the original game for a year after launch.
It sounds needlessly restrictive - why not just let players jump into Salmon Run whenever and however they want? But it's consistent with Splatoon's overall approach to multiplayer, including timed map rotations. And it should help make sure that there's a good-sized matchmaking pool whenever you actually can play.
Go off the hook with our Splatoon 2 info article and read about this weird setup for Splatoon 2 handheld-and-phone voice chat.