This example assumes the main page from which users will pop up the calendar is named Calender.aspx. The following procedure shows how to create the popup calendar:

1. In Calendar.aspx, add these controls:

<INPUT type="button" value="Pick" 
  onclick="PopupPicker('DateTextBox', 250, 250);">
<asp:linkbutton ID="Linkbutton1" runat="server"  
  OnClientClick="PopupPicker('TextBox1',
  250, 250)">LinkButton</asp:linkbutton>

2. Create a TextBox to display the selected date:

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>

3. Place the following JavaScript function in the Calendar.aspx page:

<script language=javascript>
   function PopupPicker(ctl,w,h)
   {
      var PopupWindow=null;
      settings='width='+ w + ',height='+ h + ',
      location=no,directories=no, 
      menubar=no,toolbar=no,status=no,
      scrollbars=no,resizable=no, 
      dependent=no';
      PopupWindow=window.open('DatePicker.aspx?Ctl=' + 
         ctl,'DatePicker',settings);
      PopupWindow.focus();
   }
</script>

4. Create a new .aspx page named DatePicker.aspx. This is the page that pops up when users click the calendar LinkButton.
5. Place an ASP.NET Calendar control on this page.
6. Put the following code in the Calendar's DayRender event handler:

// C#
protected void Calendar1_DayRender(
   object sender, DayRenderEventArgs e)
{
    
   HyperLink hlnk = new HyperLink();
   hlnk.Text = ((LiteralControl)e.Cell.Controls[0]).Text;
   hlnk.Attributes.Add("href", "javascript:SetDate('" + 
      e.Day.Date.ToShortDateString() + "')");
   e.Cell.Controls.Clear();
   e.Cell.Controls.Add(hlnk);
}

// VB
Protected Sub DatePicker_DayRender(
   ByVal sender As Object, ByVal e As 
   System.Web.UI.WebControls.DayRenderEventArgs) 
   Handles DatePicker.DayRender
   Dim hl As New HyperLink()
   hl.Text = CType(e.Cell.Controls(0), _
      LiteralControl).Text
   hl.NavigateUrl = "javascript:SetDate( _ 
      '" & e.Day.Date.ToShortDateString() & "');"
   e.Cell.Controls.Clear()
   e.Cell.Controls.Add(hl)
End Sub

7. Copy the following JavaScript function into DatePicker.aspx.

<script  language="javascript">
   function SetDate(dateValue)
   {
      // retrieve from the querystring the value of the Ctl param,
      // that is the name of the input control on the parent form
      // that the user want to set with the clicked date
      ctl = window.location.search.substr(1).substring(4);
      // set the value of that control with the passed date
      thisForm = window.opener.document.forms[0].elements[ctl].value = 
         dateValue;
      // close this popup
      self.close();
   }
    </script>

At this point, you should be able to click the button to pop up the calendar. When users select a date, the calendar popup window will close, and the selected date will appear in the Textbox.