add_post_meta_at_admin_column.php

<?php
/**
 * Set event month filter at Event post type admin column
 */
add_filter( 'query_vars', 'goto_calendar_register_query_vars' );
function goto_calendar_register_query_vars( $qvars ) {

	//Add these query variables
	$qvars[] = 'custom_month';
	return $qvars;

}

/**
 * Add event_date to admin column.
 */
// add event date field admin column
add_filter( 'manage_event_posts_columns', 'add_event_date_admin_columns_head' );
function add_event_date_admin_columns_head( $defaults ) {
	// Remove default post date column
	unset($defaults['date']);
	$defaults[ 'event_date' ] = 'Event Day';
	return $defaults;
}

add_action( 'manage_event_posts_custom_column', 'add_event_date_admin_column_content', 10, 2 );
function add_event_date_admin_column_content( $column_name, $post_id ) {
	global $post;
	if ( $column_name == 'event_date' ) {
		$event_date = get_post_meta( $post_id, 'event_date', true );

		if( ! empty( $event_date ) ) {
			echo date( 'Y/m/d', strtotime( $event_date ) );
		} else {
			echo '';
		}
	}
}

/**
 * Remove date filter at Event post type admin column.
 */
add_filter( 'disable_months_dropdown' , 'custom_disable_months_dropdown' , 10 , 2 );
function custom_disable_months_dropdown( $false , $post_type ) {

	$disable_months_dropdown = $false;
	$disable_post_types = array( 'event' );
	if( in_array( $post_type , $disable_post_types ) ) {
		$disable_months_dropdown = true;
	}
	return $disable_months_dropdown;

}


/**
 * Add post meta date filter to even post type.
 */
add_action( 'restrict_manage_posts', 'goto_calendar_restrict_posts_by_metavalue' );
function goto_calendar_restrict_posts_by_metavalue() {

	global $typenow;
	$months = goto_calendar_get_months();

	if ($typenow == 'event') {

		$selected = get_query_var('custom_month');
		$output  = '<select name="custom_month" class="postform">' . "\n";
		$output .= '<option ' . selected( $selected, 0, false ).' value="">' . __( 'Show All', 'some_textdomain' ) . '</option>';

		if ( ! empty( $months ) ) {

			foreach ( $months as $month ) {
				$value    = esc_attr( $month->year . '/' . $month->month );
				$month_dt = new DateTime( $month->year . '-' . $month->month . '-01' );
				$output  .= '<option value="' . $value . '"' . selected( $selected, $value, false ) . '>' . $month_dt->format( 'Y年m月' ) . '</option>';
			}

		}
		$output .= '</select>' . "\n";

		echo $output;

	}

}


add_action( 'pre_get_posts', 'goto_calendar_pre_get_posts' );
function goto_calendar_pre_get_posts( $query ) {

	//Only alter query if custom variable is set.
	$month_str = $query->get( 'custom_month' );
	if( !empty( $month_str ) ){

		$meta_query = $query->get( 'meta_query' );
		if( empty( $meta_query ) )
			$meta_query = array();

		$month = new DateTime($month_str.'/01');

		$meta_query[] = array(
			'key' => 'event_date',
			'value' => array( $month->format( 'Ymd' ), $month->format( 'Ymt' ) ),
			'compare' => 'BETWEEN',
		);
		$query->set( 'meta_query', $meta_query );

	}
}


function goto_calendar_get_months(){
	global $wpdb;
	$months = wp_cache_get( 'goto_calendar_get_months' );
	if ( false === $months ) {
		$query = "SELECT YEAR( meta_value ) AS `year`, MONTH( meta_value ) AS `month`, count( post_id ) as posts 
			FROM $wpdb->postmeta
			WHERE meta_key = 'event_date'
			AND post_id IN ( SELECT ID FROM $wpdb->posts
			WHERE post_type = 'event'
			AND post_status != 'trash' )
			GROUP BY YEAR( meta_value ), MONTH( meta_value ) ORDER BY meta_value DESC";
		$months = $wpdb->get_results($query);
		wp_cache_set( 'goto_calendar_get_months', $months );
	}
	return $months;
}