One other change, in the file apllication/views/event/viewweek.php, the line
$startday = date("d", mktime(0, 0, 0, $month, $day, $year)) - (date("N", mktime(0, 0, 0, $month, $day, $year)) % 7);//inicio de la semana
Should be replaced with
if (!cal_option("start_monday")){
$startday = date("d", mktime(0, 0, 0, $month, $day, $year)) - (date("N", mktime(0, 0, 0, $month, $day, $year)) % 7);//beginning of the week, sunday
}else{
$startday = date("d", mktime(0, 0, 0, $month, $day, $year)) - (date("n", mktime(0, 0, 0, $month, $day, $year)) % 7);//beginning of the week, monday
}