get all posts with certain meta data

I think you can do a left join :

left join 
    $wpdb->postmeta my_field_meta on (p.ID = my_field_meta.post_id and 
                                      my_field_meta.meta_key = 'subscribe') 

Where “subscribe” it’s the name of your custom field. So your code could be:

 $dayswithposts = $wpdb->get_results("SELECT DISTINCT DAYOFMONTH(p.post_date)
    FROM $wpdb->posts as p
    LEFT JOIN 
        $wpdb->postmeta my_field_meta on (p.ID = my_field_meta.post_id and 
                                          my_field_meta.meta_key = 'subscribe')     
    WHERE MONTH(p.post_date) = '$thismonth'
    AND YEAR(p.post_date) = '$thisyear'
    AND p.post_type="post" AND p.post_status="publish"
    AND p.post_date < '" . current_time('mysql') . '\'', ARRAY_N);