group posts by month/date ignore years

You can use the month parameter of WP_Query to get posts by month. For example, to get all posts published on March, ignoring year:

<?php
$the_query = new WP_Query( 'monthnum=3' );
echo '<ul>';
while ( $the_query->have_posts() ) {
    $the_query->the_post();
    echo '<li>' . get_the_title() . '</li>';
}
echo '</ul>';
wp_reset_postdata();
?>