Wait, wait, wait... wasn't Black Mesa released a few years ago? Technically yes, but also no. The current version of the long-developed remake does not include the Xen levels of the original game (and everything that follows) and it's this content that's been delayed until... a date in the future.
A post by Crowbar Collective's Adam Engels, entitled "Bad News, Good News", explains the reasons for the delay:
We are truly sorry for getting everyone's hopes up and then delaying... again. We worked very hard to make December, but we are not yet ready. As a team, we take FULL responsibility for that. We have an internal deadline we are confident in, and we will be getting everyone more details as we get closer to that date.
Engels goes on to say that "Xen has proven to be an enormous undertaking" and "it is proving to take longer than we estimated". On the bright side, the developer did share some screenshots and information about the various engine upgrades going into Xen, including god rays, image-based ambient lighting and dynamic lights.
Not much else to do except wait, which I'm sure everyone's pretty good at by now.
Bad News, Good News [Steam]