Hi,
I have changed the message in control
template of RadScheduleView in a Grid named AppointmentSourceWarning but it still shows the message "Please provide an AppointmentsSource".
I have changed it in officeblack, officeblue, ExpressionDark, OfficeSilver, Summer Themes etc.
I prepared and attached a sample project for you with an edited warning message. In the project i have used Implicit Styling, you can read this article about the benefits that Implicit Styling gives.