see the code
Code:
 <tr>
                <td>
                   End Date of Conference</td>
                <td>
                    <asp:TextBox ID="txt_enddays" runat="server" onchange="checkdate(this);"></asp:TextBox> 
                       <a href="javascript:calendar_window=window.open('calendar.aspx?form1=form1.txt_enddays','calendar_window','width=554,height=488');calendar_window.focus()">

                                                <img alt=""  src="images/calendar.png"   />  
                                                </a> 
                          </td>
            </tr>

Code:
<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="calendar.aspx.vb" Inherits="gfln1.calendar_aspx" %>

<!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></title>
     <script runat="server">
    Protected Sub Calendar1_DayRender(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.DayRenderEventArgs) Handles Calendar1.DayRender 
                  If e.Day.Date < DateTime.Today.Date Then 
                             e.Cell.Enabled = False        
                             e.Cell.ToolTip = "This date is not available"                      
                 End If 
    End Sub

Private Sub Calendar1_SelectionChanged(sender As Object, e As EventArgs)
    Dim strjscript As String = "<script language=""javascript"">"
    
        strjscript &= "window.opener." & _
              HttpContext.Current.Request.QueryString("form1") & ".value = '" & Calendar1.SelectedDate & "';window.close();"
        strjscript = strjscript & "</script" & ">" 'Don't Ask, Tool Bug

        Literal1.Text = strjscript  'Set the literal control's text to the JScript code

End Sub

</script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
     <asp:Calendar id="Calendar1" runat="server" 
                     OnSelectionChanged="Calendar1_SelectionChanged" 
                    
                     ShowTitle="true" DayNameFormat="FirstTwoLetters" 
                     SelectionMode="Day" BackColor="#ffffff" 
                     FirstDayOfWeek="Monday" BorderColor="#000000" 
                     ForeColor="#00000" Height="60" Width="120">
        <TitleStyle backcolor="#000080" forecolor="#ffffff" />
        <NextPrevStyle backcolor="#000080" forecolor="#ffffff" />
        <OtherMonthDayStyle forecolor="#c0c0c0" />
    </asp:Calendar>
    <asp:Literal id="Literal1" runat="server"></asp:Literal>
</div>
    </form>
</body>
</html>
Error: window.opener.form1 is undefined
Source File: http://localhost:2107/calendar.aspx?....txt_startDays
Line: 44

any help