Hi,
Try this Hope Itwill work,befor doing this add ajax dll and add script manager in design page
Hope it will work...
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" EnablePageMethods="true">
</asp:ToolkitScriptManager>
<asp:TextBox ID="txtCloseDate" runat="server"></asp:TextBox>
<asp:CalendarExtender
ID="CalendarExtender2" runat="server" PopupButtonID="imgCal1" TargetControlID="txtCloseDate"
Enabled="True">
</asp:CalendarExtender>
<asp:Image ID="imgCal1" runat="server" ImageUrl="~/Images/cal1.jpg" />