As far as I understand you, you want to tooltipify a specific HTML element of the Calendar control. In this case, you have to add the ID of the HTML element (IsClientId=true as this is a ClientID) to the TargetControls collection of the RadToolTipManager. When the manager is initialized, it will find the element with that ID, and tooltipify it. The only case, when you do not get a tooltip, is the case where you have specified the wrong ID. In case you prepare and send us a simple test page, demonstrating your setup, I can check if my assumption is correct, or in case it is not, research what is the real cause for the problem.
the Telerik team