publish-hidden-post-after-one-month.php

<?php
add_action( 'publish_post_after_month', 'musilda_publish_post_after_month' );
function musilda_publish_post_after_month() {
  $post_id = 123;
	wp_publish_post( $post_id );
}

add_action( 'init', 'musilda_single_event' );
function musilda_single_event(){
  wp_schedule_single_event( 30 * DAY_IN_SECONDS, 'publish_post_after_month' );
}