There are two issues with your code:
1. First, the GroupHeader contains a button which switches to the DayViewDefinition when clicked. So in order to handle the Click event, you should remove the Command to this button. You should customize the GroupHeaderStyleSelector in order to achieve this. In the attached project I demonstrated the approach. Please check the CustomGroupHeaderTemplate template where the command to the GroupHeaderButton is removed.
2. You should handle MouseDown event ot the TextBlock as Click event is not available. Also the TestCommand should be bound like this:
I've attached a simple project with the needed changes. Hope it helps.
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>