Hi El Neilo,
The message "Please provide an AppointmentsSource" is in the control template of RadScheduleView in a Grid named AppointmentSourceWarning. It is easily accessible and modifiable.
However, for the purpose of your scenario, I would suggest that you use another approach instead. You can take advantage of the RadBusyIndicator control to show a loading message and animation till your appointmetns load. Integration between RadScheduleView and RadBusyIndicator is done via the BackgroundWorker class. You can see this approach realized in the FirstLook example of RadBusyIndicator
I hope this helps.
All the best,
the Telerik team
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