Show related (archive) month and year to post

Use following code after the loop:

$month_num = get_the_date('n');
$month_txt = get_the_date('F');
$year = get_the_date('Y');

// change 'posts_per_page' to any value you need, '-1' means 'all'
query_posts( 'posts_per_page=-1&monthnum='.$month_num.'&year=".$year );

echo "<p class="">Archived in '.$month_txt.', '.$year.'</p>';
echo '<ul>';

while ( have_posts() ) : the_post();
    echo '<li><a href="'.get_permalink().'">'.get_the_title().'</a></li>';
endwhile;

echo '</ul>';
wp_reset_query();

It’s not tested, but you got the idea.