The bug appeared on iOS 6 devices on January 1st.
Apparently, many users of iOS 6 devices making use of the Do Not Disturb found a problem with the feature: it could no longer be disabled automatically.
Apple acknowledged the issue and announced that the bug will correct itself next week.
“Do Not Disturb scheduling feature will resume normal functionality after January 7, 2013,” the company said in a support article. “Before this date, you should manually turn the Do Not Disturb feature on or off.”
So, on January 7 everything should return to normal.