I don't really like this idea.
My first concern is the obvious -- spam and useless posts will spike during this time period.
The second concern is that activity that would normally be spread out will be condensed into this time period. As in, there will be a dead zone gap in posting immediately proceeding such a time. People will deliberately hold back their posting until this time.
My third concern is that it's unfair to people who are unable to come online to post at that time. In addition to people who are just too busy to show up or have other commitments, it might occur at hours that happen to be extremely inconvenient for people living in varying time zones,
I think this said it all.
None.