custom query not working when search by custom field

Hi All,
I’ve got the below code.

$querystr = "
    SELECT wposts.*
    FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
    WHERE wposts.ID = wpostmeta.post_id
    AND wpostmeta.meta_key = 'Day'
    AND wpostmeta.meta_value = '5'
    AND wposts.post_status = 'publish'
    AND wposts.post_type = 'post'
    ORDER BY wposts.post_date DESC
 ";

The above works but I need to add meta key Month and eventually year. So to start I added month. As soon as I do, it stop working. Whats wrong with the below? Am baffled!

$querystr = "
    SELECT wposts.*
    FROM $wpdb->posts wposts, $wpdb->postmeta wpostmeta
    WHERE wposts.ID = wpostmeta.post_id
    AND wpostmeta.meta_key = 'Day'
    AND wpostmeta.meta_value = '5'
    AND wpostmeta.meta_key = 'Month'
    AND wpostmeta.meta_value = '8'
    AND wposts.post_status = 'publish'
    AND wposts.post_type = 'post'
    ORDER BY wposts.post_date DESC
 ";

many thanks