Hi,
I give you simple example working calender extender controls a single page.
Add AjaxControlToolkit control on web.config file.
<pages>
<controls>
<add tagprefix="asp" namespace="System.Web.UI" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
<add tagprefix="asp" namespace="System.Web.UI.WebControls" assembly="System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
<add tagprefix="ajax" namespace="AjaxControlToolkit" assembly="AjaxControlToolkit" />
</controls>
</pages>
Add Default.aspx
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Multiple Controls Ajax
</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
<table>
<tr>
<td>
Date1
</td>
<td>
:
</td>
<td>
<asp:TextBox ID="txtDate1" runat="server">
<ajax:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="txtDate1"
CssClass="cal_Theme1" Format="dd-MMM-yyyy">
</td>
</tr>
<tr>
<td>
Date2
</td>
<td>
:
</td>
<td>
<asp:TextBox ID="txtDate2" runat="server">
<ajax:CalendarExtender ID="CalendarExtender2" TargetControlID="txtDate2" CssClass="cal_Theme1"
runat="server" Format="dd-MMM-yyyy">
</td>
</tr>
<tr>
<td>
Date3
</td>
<td>
:
</td>
<td>
<asp:TextBox ID="txtDate3" runat="server">
<ajax:CalendarExtender ID="CalendarExtender3" runat="server" TargetControlID="txtDate3"
CssClass="cal_Theme1" Format="dd-MMM-yyyy">
</td>
</tr>
<tr>
<td>
Date4
</td>
<td>
:
</td>
<td>
<asp:TextBox ID="txtDate4" runat="server">
</td>
<ajax:CalendarExtender ID="CalendarExtender4" runat="server" TargetControlID="txtDate4"
CssClass="cal_Theme1" Format="dd-MMM-yyyy">
</tr>
</table>
</div>
</form>
</body>
</html>