TargetControlID
is a property in Ajax Calender Extender.You have to give ID of a Textbox.It's must to use this property for this control.
Eg;
<asp:textbox id="TextBox1" runat="server"></asp:textbox>
<cc1:calendarextender id="CalendarExtender3" runat="server" targetcontrolid="TextBox1">
</cc1:calendarextender>