List events by month

Perhaps there are better optimized ways, but this is how I would do it:

1. Get all Events by using WP_Query, and sort them by your custom date field
2. Loop through all of them and create a new multidimensional array
3. In this array we are grouping all posts by their date (month and year)
4. And lastly, loop through our new array, to get the dates and events we extracted

Here’s the code:

<?php
// Edit these values
$custom_post_type="event";
$custom_date_field = 'event_date';
$order="ASC"; // from the oldest to the newest

// query
$the_query = new WP_Query( [
    'post_type'      => $custom_post_type,
    'posts_per_page' => - 1,
    'meta_key'       => $custom_date_field,
    'orderby'        => 'meta_value',
    'order'          => $order,
] );

// We are creating new multidimensional array
$all_events = [];

while ( $the_query->have_posts() ) :
    $the_query->the_post();

    $date       = strtotime( get_post_meta( get_the_ID(), $custom_date_field, true ) );
    $month_year = date( "F Y", $date );

    $all_events[ $month_year ][] = $the_query->post;

endwhile;
wp_reset_query();

// And to print this:
foreach ( $all_events as $month_year => $events ) : ?>
    <h3><?php echo $month_year ?></h3>
    <ul>
        <?php 
        /** @var \WP_Post $event */
        foreach ( $events as $event ) : ?>
            <li><?php
                // post title
                echo $event->post_title;
                // any custom field
                the_field( 'acf_custom_field', $event->ID );
                ?></li>
        <?php endforeach; ?>
    </ul>
<?php endforeach; ?>

You can do whatever you want inside the second foreach loop. Edit it as you need.