//w.php
  <td>
            <input type="checkbox" name="day[]" value="월">월  
            <input type="checkbox" name="day[]" value="화">호  
            <input type="checkbox" name="day[]" value="수">수  
            <input type="checkbox" name="day[]" value="목">목  
            <input type="checkbox" name="day[]" value="금">금  
            <input type="checkbox" name="day[]" value="토">토
        </td>

//send.php
    // validation expected data exists
    if(!isset($_POST['f_name']) ||
        !isset($_POST['f_tel1']) ||
        !isset($_POST['f_tel2']) ||
        !isset($_POST['f_tel3']) ||
        !isset($_POST['email']) ||
        !isset($_POST['ord_addr1']) ||
        !isset($_POST['day']) ||
        !isset($_POST['comments'])) {
        died('We are sorry, but there appears to be a problem with the form you submitted.');      
    }
     
 
    $f_name = $_POST['f_name']; // not required
    $f_tel1 = $_POST['f_tel1']; // required
    $f_tel2 = $_POST['f_tel2']; // required
    $f_tel3 = $_POST['f_tel3']; // required
    $ord_addr1 = $_POST['ord_addr1']; // required
    $email_from = $_POST['email']; // required
    $list = implode(', ', $_POST['day']); //수정한곳
    $comments = $_POST['comments']; // required
$email_message .= "요일: ".clean_string($list)."\n\n";