its very simple to create
follow this code your easily get the answer
/* First Of all Include ToolKit Script Manager */
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</asp:ToolkitScriptManager>
/* Text Box To take Value From Calendor Extendor
<asp:TextBox ID="txtDateFrom" runat="server"></asp:TextBox>
/* Image Button on click of which You want to pop Calendor
<asp:ImageButton ID="btnDate2" runat="server" ImageUrl="~/Admin/Images/CalendorIcon.jpg" Width="4%" />
/* Last Your Calendor Extendor
<asp:CalendarExtender ID="CalendarExtender1" TargetControlID="txtDateFrom" PopupButtonID="btnDate2" runat="server">
</asp:CalendarExtender>