From the update post by @steemitblog...

Steemit is proud to announce a major upgrade to the Steem protocol today. It is our desire to give everyone in the community ample time to review the code and upgrade their software before any hard-fork occurs. Our release process will go as follows:

Publish Release Candidate (Today)
Confirm Release in one week (9/9/2016)
Protocol changes effective 9/13/2016 at 11 AM EST (3 PM GMT)