Efullcalendar FirstDay Property not working

Hi! have a nice day everyone!
I can’t get the property “FirstDay” to work, always starts on “Dom” (which is Sunday)
This is my code;
Thanks for your help!

$this->widget('ext.EFullCalendar.EFullCalendar', array(
  'lang'=>'es',
'options'=>array(
 'firstDay' => '4',                               
   'timeFormat'=>'HH:mm{ - HH:mm}',
'events'=>$this->createUrl('calendario/calendarEvents'),
	'header'=>array(
		'left'=>'prev,next',
		'center'=>'title',
		'right'=>'month,basicWeek,agendaList',
	)
)));