Use Following Code.
JavaScript Code :
function checkDate(sender, args)
{
var myDate = new Date();
myDate.setDate(myDate.getDate() + 1);
myDate.setHours(0, 0, 0, 0);
if (sender._selectedDate <= new Date(myDate))
{
alert("You cannot select a day earlier than tomorrow !");
sender._selectedDate = new Date( myDate.setDate(myDate.getDate() + 1));
sender._textbox.set_Value(sender._selectedDate.format(sender._format))
}
}
HTML Code:
<asp:textbox id="TextBox1" runat="server" xmlns:asp="#unknown"></asp:textbox>
<cc1:calendarextender id="CalendarExtender1" xmlns:cc1="#unknown">
runat="server" OnClientDateSelectionChanged="checkDate" TargetControlID="TextBox1" />
</cc1:calendarextender>
Hope it will work for you.