If u use calenderExtenderextender control.. enter your code like this at script tag...u cal this method in calenderextender control add this properity also...
OnClientDateSelectionChanged="checkDate"
<pre lang="xml"><script type="text/javascript">
function checkDate(sender, args) {
if (sender._selectedDate > new Date()) {
alert("You cannot select a future date!");
sender._selectedDate = new Date();
// set the date back to the current date
sender._textbox.set_Value(sender._selectedDate.format(sender._format))
}
}</pre>