1.12 update for Minecraft was supposed to be released today but Nathan Adams (@Dinnerbone) Tweeted this five hours ago.
Unfortunately we will not be able to release Minecraft 1.12 today. Sorry 😦 The new plan is to aim for Wednesday (7th). We’ll let you know! -Nathan Adams @Dinnerbone Via Twitter
The update was postponed until the 7th. It is unknown why they are postponing the update.
He also posted this:
In lighter news, the current pre-release actually seems very stable and is practically the same version we will release on Wednesday! 🙂 -Nathan Adams @Dinnerbone Via Twitter