gistfile1.phtml

<?php
echo $this->Form->input('control_frequency_id', array('id' => 'frequency'));
echo $this->Form->input('name', array('id' => 'week_day')); 
?>

<script type="text/javascript">

    $('#frequency').hide();
    {
        selection = $(this).val();    
           switch(selection) { 
             case 'Weekly':
               $('#week_day').show();
             break;
             default:
               $('#week_day').hide();
             break;
   }
});
</script>