<?php

$dateTime = DateTime::createFromFormat('d M Y', '12 Dec 2016');
 
function getWorkingDayAfterNDays(DateTime $dateTime, $n) {
    if ($n < 0) {
        throw new Exception("Число должно быть положительным {$n}");
    }
 
    while (true) {
        $dateTime->modify('+1 day');
        if (isWorkingDay($dateTime)) {
            $n--;
            if ($n === 0) {
                return $dateTime;
            }
        }
    }
}

function isWorkingDay(DateTime $dateTime) {
    $workDays = [
        '19-12-16',
        '20-12-16',
        '23-12-16',
        '24-12-16',
        '25-12-16',
        '26-12-16',
    ];
 
    return in_array($dateTime->format('d-m-y'), $workDays);
}

var_dump(getWorkingDayAfterNDays($dateTime, 5));