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(
 'firstDay' => '4',                               
   'timeFormat'=>'HH:mm{ - HH:mm}',

Could you share information about EFullCalendar?

I have not been able to work with him and I need it to verify appointments in an application.

I thank you from now

Hi, it is https://fullcalendar.io/