$text="
BEGIN:VCALENDAR
VERSION:1.0
BEGIN:VEVENT
CATEGORIES:MEETING
STATUS:TENTATIVE
DTSTART:".$startDateTime."
DTEND:".$endDateTime."
SUMMARY:Interview for the candidate".$cname."
DESCRIPTION:".$message."
CLASS:PRIVATE
END:VEVENT
END:VCALENDAR";

$mail->SetFrom('xxxxxx@yahoo.com', 'xxxx');
$mail->IsSMTP();
$mail->Host = "smtp.mail.yahoo.com";

$mail->SMTPAuth = true;
$mail->Username = 'xxxxxxxx@yahoo.com';
$mail->Password = 'xxxxx';


$mail->AddAddress($addresses[$i]);
$mail->Subject    = "Interview schedule of Candidate";

    $headers = "From: Sendern";
        $headers .= "Reply-To: xxxxxx@yahoo.comn";
        $headers .= "MIME-Version: 1.0n";
        $headers .= "Content-Type: text/calendar; method=REQUEST; charset=utf-8n";
        $headers .= "Content-Transfer-Encoding: 8bitn";
        $headers .= "Content-class: urn:content-classes:calendarmessagen";
$mail->Body=$body;

if(!$mail->Send($headers,$body))
{
        echo "Mailer Error: " . $mail->ErrorInfo;
}
else
{
    echo "Message sent!";
}

$mail->IsHTML(FALSE);

$mail->Subject = $name;
$mail->Body = $description;
$mail->AltBody = $body; // in your case once more the $text string
$mail->Ical = $message; // ical format, in your case $text string