Thank you for the question.
To add a tooltip to the day button first you need to create a custom style for the button and then create the tooltip. Attached, you will find a simple project which illustrates this approach. There you can customize the text of the tooltip by binding the TextBlock's Text
property and creating a converter, with which to format the tooltip. Another approach to formatting the text of the tooltip is to create an attached behavior for the tooltip text.
If you need further assistance please feel free to contact us again.
All the best,
the Telerik team