There is not such option for RadCalendar, as the month views are built on the fly, they are not initially available. One option would be to set AutoPostBack="true" for the calendar, and on the server DayRender event disable/enable dates based on DB values. To prevent the page from reloading on navigation, you could wrap the RadCalendar in a RadAjaxPanel or add it to a RadAjaxManager setting where it updates itself.
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