According to the official Halo Twitter account, the release date listed for Halo 3: ODST remastered on Xbox.com was a placeholder.

A placeholder release date for the ODST campaign for Halo: The Master Chief Collection was inadvertently listed on the Xbox Store. (1/2)
— Halo (@Halo) May 27, 2015

We continue final testing and will make an official announcement once our release date is determined, and apologize for any confusion. (2/2)
— Halo (@Halo) May 27, 2015

The original story is below.

Microsoft has listed the release date for its remastered version of Halo 3: ODST for Xbox One as May 29.

The date, which has yet to be officially confirmed by Microsoft or Halo stewards 343 Industries, appeared on Xbox.com, but has since been removed.

The re-release, which was announced as recompense for significant matchmaking issues in Halo: The Master Chief Collection, will be available for free to those that played Master Chief Collection between its release date (November 11) and December 19.

At the time, head of 343 Industries Bonnie Ross said the developer is giving these items to players for free to thank them for sticking with Halo: The Master Chief Collection during its troubled launch.

"This has been a humbling experience and highlighted how we as a studio can – and need – to do better for Xbox fans around the world," Ross said. "We are so grateful to our fans who have stood by our side and we appreciate all of your patience as we worked through these issues."

Previously, 343i said the updated version of Halo 3: ODST's campaign and a reimagined version of the Halo 2 multiplayer map Relic should be added to Halo: The Master Chief Collection in May, but offered nothing more specific than that window.