day_ru.php

<?php
$trans = array("Monday" => "Понедельник",
               "Tuesday" => "Вторник",
               "Wednesday" => "Среда",
               "Thursday" => "Четверг",
               "Friday" => "Пятница",
               "Saturday" => "Суббота",
               "Sunday" => "Воскресенье"
);

return strtr($input, $trans);