If you're using mysql, you're looking to add in your statement:
WHERE MONTH(User.date) = MONTH(CURRENT_DATE - INTERVAL 1 MONTH)
I believe you can do this in cakePHP by doing the following:
$last_month_users = $this->User->find('all', array(
'conditions' => array(
'MONTH(User.date) = MONTH(CURRENT_DATE - INTERVAL 1 MONTH)'
)
));