The expanded cut of the movie, which features more than 17 minutes of extra material, largely centering on the characters of Rogue (Anna Paquin) and Magneto (Sir Ian McKellen) in a bleak alternate future, was originally announced as a potential digital exclusive but now is expected to be released on DVD and Blu-ray as well.
Typically, Singer makes announcements on his personal Instagram and Twitter pages. It may be that this announcement is considered "big enough" that the official account simply wanted to draw more attention to it.
So far, there's no firm release date for the X-Men: Days of Future Past Rogue Cut, other than this summer. The announcement will likely be the release date, and possibly some more specifics about the differences between this film and the theatrical version that was released on Blu-ray last year.