A new Apple support doc advises that "Do Not Disturb scheduling feature will resume normal functionality after January 7, 2013. Before this date, you should manually turn the Do Not Disturb feature on or off.
To turn off the scheduling feature, tap Settings > Notifications > Do Not Disturb and switch Scheduled to Off."
iOS 6 added new calling features to your iPhone. Now when you decline an incoming call, you can instantly reply with a text message or set a callback reminder. And when things are just too hectic, turn on Do Not Disturb and you won't be bothered by anyone - except can't-miss contacts like your boss or your better half. The feature works to suppress all incoming calls and notifications and can be enabled manually or scheduled at a recurring time.