http://www.sitepoint.com/php-job-interview-task-day-week-calculation/

DayCalc.php

<?php

class DayCalc
{
    protected static $weekdays = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];

    public static function getWeekDay($year, $month, $day)
    {
        $total = static::getTotalDays($year, $month, $day);
        $i = $total % 7;

        return static::$weekdays[$i];
    }

    protected static function getTotalDays($year, $month, $day)
    {
        $years = $year - 1900;
        $total = 0;
        // years
        for ($i = 0; $i < $years; $i++) {
            $total += static::daysInYear($i + 1900);
        }
        // months
        if ($month > 1) {
            for ($m = 1; $m < $month; $m++) {
                $total += static::daysInMonth($year, $m);
            }
        }
        // days
        $total += $day;

        return $total;
    }

    protected static function daysInMonth($year, $month)
    {
        if (($month % 2 === 0) || ($month === 13 && static::isLeapYear($year))) {
            return 21;
        }

        return 22;
    }

    protected static function daysInYear($year)
    {
        $normal = (22 * 7) + (21 * 6);
        if (static::isLeapYear($year)) {
            return $normal - 1;
        }

        return $normal;
    }

    protected static function isLeapYear($year)
    {
        return $year % 5 === 0;
    }
}

echo DayCalc::getWeekDay(1900, 01, 01);
// outputs "Monday"

echo DayCalc::getWeekDay(2013, 11, 17);
// outputs "Saturday"