There is a crescent moon shape next to my time display in top border of Ipad3. What is it?
iPad (3rd gen) Wi-Fi, iOS 6.1.1
That would be the "Do not disturb" option.
That is the DND, or Do not disturb option. It silences all incoming calls, text messages, emails, alerts etc.. when the device is locked and in sleep mode.
Go to Settings->Do Not Disturb. You can turn it off or on.
It can also be scheduled to automatically turn on and off, in Settings->Notifications->Do Not Disturb
Additionally The iPad Guide user guide also mentions this
There is a crescent moon shape next to my time display in top border of Ipad3. What is it?
