<!DOCTYPE html>
<html>
<head>
<script  runat="server">
Sub DaySelect(obj As Object, e As DayRenderEventArgs)
      If e.Day.IsSelected Then
      Label1.Text = e.Day.DayNumberText
      End If
End Sub
</script>

</head>
<body>
      <form runat="server">
      <asp:Calendar id="cal1" runat="server" OnDayRender="DaySelect"/>
      The selected date is:
      <asp:Label id="Label1" runat="server"/>
</form>
</body>
</html>