THis section allow you to show calendaer when you click in textbox using ajax toolkit:

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>

<!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>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">

<asp:ToolkitScriptManager runat="Server" EnableScriptGlobalization="true"
EnableScriptLocalization="true" ID="ScriptManager1" />

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<asp:CalendarExtender ID="CalendarExtender1" runat="server" TargetControlID="TextBox1" Format="dd/MM/yyyy">
</asp:CalendarExtender>


</form>
</body>
</html>


Thanks