This message is sent when a new event is created and the event is already approved, AND:
The person creating the event is an approver.
The person creating the event has the permissions "Events are approved when entered" and the permission "Mail approvers when this user or group adds an event".
Disabling these Messages
Login to the calendar, and select the Toolbox Icon | Preferences from the menu. Check the preference "Disable Empty Approval Messages" and save changes.