Friquency
            <select name="frequency">
			<?php
			    $frequency = array("First","Second","Third","Fourth","Last","Every");
			    $frequ_val = array("1","2","3","4","Last","Every");
			        for($fr=0; $fr<6; $fr++){
			        echo "<option value=\"".$frequ_val[$fr]."\">".$frequency[$fr]."</option>"; 
			    }
			?>
            </select>

$frequency = isset($_POST['frequency'])?($_POST['frequency']):'';

if($_POST['frequency']=="Last")
	        {
				echo ""; - // вот тут должна быть проверка и посыл аргумента.
	        }
	    		elseif($_POST['frequency']=="Every")
	        {
	            $peremennaja = $_POST['0'];  // Тут тоже проверка и каркуляция. Ноль - не правильно.
	        }
	   			else	
	        {
	            $peremennaja = $_POST['frequency'];
	        }

Friquency
            <select name="frequency">
			<?php
			    $frequency = array("First","Second","Third","Fourth","Last","Every");
			    $frequ_val = array("1","2","3","4","6","7");
			        for($fr=0; $fr<6; $fr++){
			        echo "<option value=\"".$frequ_val[$fr]."\">".$frequency[$fr]."</option>"; 
			    }
			?>
            </select>

Query Monthly

SELECT  `description` AS  'Task', DATE_FORMAT( current_date,  '%d %b %y' ) AS  'Current date',  `start_time` AS  'Time'
FROM tasks
WHERE  `code` LIKE  'monthly'
AND  `start_time` > current_time
AND  `exp_day` >= current_date
AND  `day_of_week` = DAYOFWEEK( current_date ) 
AND  `month_of_year` = MONTHNAME( current_date ) 
AND `frequency` = WEEKDAY (current_date)
ORDER BY  `start_time`