"Quiet Hours" is a free download, but the scheduler requires an in-app purchase. And the app allows you to set the volume on the phone after the do not disturb feature is no longer in effect.
The Apple iPhone has its own Do Not Disturb feature as part of iOS 7, and Android users can install an app by the very same name from the Google Play Store. While both the iOS and Android applications offer a way for users to designate special contacts that can still get the phone to ring during silent time periods, the Windows Phone app does not offer this. Perhaps the developer, Amaze Pictures, will include this with the next update.
source: QuietHours via WMPoweruser