Halo: The Master Chief Collection does not yet have a release date, and when it gets one it will actually be six. That's because each game in the series will be released individually, beginning with Halo: Reach and then progressing fiction-chronologically through Combat Evolved, 2, 3, ODST, and 4. We know that the first of them will drop sometime this year, however—343 said so in the announcement—and it's possible that the entire series could be out before 2019 wraps up.
That little bit of optimism comes by way of the new Splash Damage newsletter, helpfully posted to Resetera. "The cat is out of the bag. We're incredibly proud to be working closely with Microsoft once more, this time on bringing Halo: The Master Chief Collection to PC," it says. "Halo: Reach is coming first, with the rest of the titles scheduled throughout the year."
I suppose you could take that a few ways—announcing in September that ODST is scheduled for release in 2022, for instance—but the general feeling is that it means all of the games in the collection will roll out this year. With just nine full months left in 2019, that doesn't leave a lot of time to spin things up, although it's reasonable to assume that 343 and company didn't just start working on it after the announcement.
343 community director Brian Jarrard declined to clarify the statement, writing on Reddit, "As we've said, 'they're ready when they're ready' but happy to hear Splash Damage is as eager as we are to get these into your hands!" But he also allowed that things could possibly—possibly—happen fairly quickly.
"Reach will be the first title out of the gate and while I'm not at liberty to give you hard details yet, I think it's safe to say that flighting will be kicking off sooner than people probably expect," he wrote in a separate post. "Remember, the outcomes of public flighting will play a large part in driving the retail release. If things are going great, it'll be swift. If we encounter unforeseen issues then we'll have to spend more time."
He also delivered some calming words to those who noticed the appearance, and then sudden disappearance, of the "coming soon" on the Steam listing. 343 is "checking into it," he said, but "nothing has changed though, everything with MCC on PC is still proceeding exactly as planned."