I have a theory about what might happen on September 24th, 2013. I apologize in advance for not including all the needed links to back up my theory, but the information that has influenced my thoughts can easily be found online.
To start off, apparently, on the days of recent tragic events (i.e. Sandy Hook, Boston Bombing), there have been safety drills scheduled close to the areas of the tragedies. These safety drills have been strangely linked to the tragedies that occured. For example, on the day of the Sandy Hook tragedy, there was an Active Shooter Drill scheduled in Bridgeport, Connecticut, from 9am - 4pm - around the time of the shooting, around the same area. The drill's focus was "planning for the needs of children in disasters." For another example, the Boston bombing happened on the same day, in the same area as the scheduled Controlled Explosion Drill. Here are some more examples of drills that have turned into real events of terror:
On September 24, 2013, a Hostile Action Based (HAB) Exercise with NRC Participation is scheduled from late afternoon to the late evening hours in Osage County, MO. Here is the source:
I haven't had much time to research what type of incident a Hostile Action Based Exercise is geared towards preventing/teaching how to handle, but to my understanding, it could have to do with something nuclear. September 24, 2013 is going to be the 50th anniversary of the (nuclear) Test Ban Treaty being ratified by the United States, and the 17th anniversary of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) being opened for signature. The choice to start the countdown on the 77th day has also been speculated to be linked to the book 77 Days In September, which takes place in a post-nuclear explosion United States. The book's location, however, is in Dallas, TX.
In conclusion, the pronunciationbook YouTube channel site may be warning us of an impending nuclear disaster in Osage County, MO.