http://codex.wordpress.org/Class_Reference/WP_Meta_Query
if (isset($_GET['month'])):
$args = array(
'post_type' => 'any',
'meta_query' => array(
array(
'key' => 'YourAcfDateFieldKey',
'value' => $_GET['month'],
'compare' => 'LIKE'
)
)
);
$myposts = get_posts($args);
endif;
try something like that