If you have PHP 5.3.+ on the server, replace function getWeekSelect in the file administrator/components/com_booking/helpers/html.php near line 657 with following code :
function getWeekSelect($name, $selectedWeek, $selectedYear, $deep, $attribs = '')
    {
    	$arr = array();
		for ($i = 0; $i <= $deep; $i ++) {
			$date = JFactory::getDate('+ ' . $i . ' week');
			$date->modify('this week');
 
			$week = (int) $date->format('W');
			$week1 = $date->format("j.n.y");
			$week2 = $date->modify('+6 days')->format("j.n.y");
			$year = (int) $date->format('Y');
			$text = $week1 . ' - ' . $week2;
			$arr[] = JHTML::_('select.option', $week . ',' . $year, $text);
		}
        return JHTML::_('select.genericlist', $arr, $name, $attribs, 'value', 'text', $selectedWeek . ',' . $selectedYear);
    }