The purpose of the calendar is primarily for selecting dates and it allows some customization. Although such a scenario is possible, it will involve some advanced customization and the solution would not feel very "native" to the control.
Here is sample outline:
You can try adding a custom DataTemplate through the DayTemplate or DayTemplateSelectors properties of the calendar. This data template will be bound to a CalendarButtonContent class. In the template, you can bind the DataContext of (lets say) an ItemsConrol using a Converter that will return a list of appointments, based on the given date.
I can provide more details if you decide to go this way.
All the best,
the Telerik team