The DayRender event is the right place where you could perform calculations on every day from the current month view. However handling the DayRender event and accessing the table cell of the calendar in not performance costing operation. Also based on the provided information it is hard to say what is causing the described issue on your end. Could you please post your code behind file. Thus we will be able to get more information about your scenario and provide you more to the point answer.
Looking forward for your reply
All the best,
the Telerik team
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