Lato client:
  <script type="text/javascript" src="FancyBox/lib/jquery-1.8.0.min.js"></script>

  <!-- Add mousewheel plugin (this is optional) -->
  <script type="text/javascript" src="FancyBox/lib/jquery.mousewheel-3.0.6.pack.js"></script>

  <!-- Add fancyBox main JS and CSS files -->
  <script type="text/javascript" src="FancyBox/source/jquery.fancybox.js"></script>
  <link rel="stylesheet" type="text/css" href="FancyBox/source/jquery.fancybox.css" media="screen" />


    <link href="StyleSheet.css" rel="stylesheet" type="text/css" />
    <script type="text/javascript">

        $("#prova").fancybox({
          openEffect  : 'elastic',
        closeEffect  : 'elastic',
          helpers : {
        title : {
          type : 'float'
                }
          },
            maxWidth: 450,
            maxHeight: 550,
            fitToView: false,
            width: '100%',
            height: '100%',
            autoSize: false,
            type: 'iframe'
        });
</script>
<asp:Button ID="prova" runat="server" Font-Size="8pt" 
Height="20px" TabIndex="1" Text="Visualizza presenze" ValidationGroup="ricerca_presenze" Width="120px" href="presenze_dettaglio.aspx?data=<% data_presenza %>"
title="Elenco presenti nella struttura alla data di lavoro selezionata"/>

lato server (vb):

    Protected data_presenza As String
    data_presenza=txt_arrivo.text


mentre nella seconda pagina aspx ecco il codice:

    Protected Sub Page_LoadComplete(sender As Object, e As System.EventArgs) Handles Me.LoadComplete
        Albergo = CType(Session("Albergo"), iAlbergo)
        If Not Page.IsPostBack Then
            Dim data As String = Request.QueryString("data")
            gw_Presenze.DataSource = CreateDataSource(Albergo.CodAlbergo, data)
            gw_Presenze.DataBind()
        End If
    End Sub