/* 月別・日別アーカイブにカスタム投稿を含めて表示する
---------------------------------------------------------------- */
function my_pre_get_posts( $query ) {
    if ( ($query->is_month() || $query->is_day()) && $query->is_main_query() ) {
        $query->set( 'post_type', array('post','news', 'gallery') );
    }
}
add_action( 'pre_get_posts', 'my_pre_get_posts' );

function my_getarchives_where( $where ){
    $where = "WHERE";
    $where .= " (post_type = 'post' OR post_type = 'news' OR post_type = 'gallery')";
    $where .= " AND post_status = 'publish'";
    return $where;
}
add_filter( 'getarchives_where', 'my_getarchives_where' );

/*
 * 複数のカスタム投稿タイプの記事をアーカイブ・カレンダーに表示
 * （WP 6.8.0以降のバージョンに対応）
 *
 * 呼び出し方法：
 *	＜？php my_get_calendar();？＞
 *		または
 *	＜？php my_get_calendar(array('initial'=>true, 'display'=>true, 'post_type'=>'post'));？＞		
 */

/* アーカイブ・カレンダーに複数のカスタム投稿を追加
 * WordPress Developer Resources (functions) - get_calendar()：
 * https://developer.wordpress.org/reference/functions/get_calendar/
---------------------------------------------------------------- */
function my_get_calendar( $args = array() ) {
	global $wpdb, $m, $monthnum, $year, $wp_locale, $posts;

	//追加したいカスタム投稿タイプを変数に格納
	$custom_post_type = "post_type = 'news' OR post_type = 'gallery'";

	// 以下の行を有効化すると、標準の'get_calendar()' と同様の動作になります。
  // $custom_post_type = "post_type = ''";

	$defaults = array(
		'initial'   => true,
		'display'   => true,
		'post_type' => 'post',
	);

	$original_args = func_get_args();
	$args          = array();

	if ( ! empty( $original_args ) ) {
		if ( ! is_array( $original_args[0] ) ) {
			if ( isset( $original_args[0] ) && is_bool( $original_args[0] ) ) {
				$defaults['initial'] = $original_args[0];
			}
			if ( isset( $original_args[1] ) && is_bool( $original_args[1] ) ) {
				$defaults['display'] = $original_args[1];
			}
		} else {
			$args = $original_args[0];
		}
	}

	/**
	 * Filter the `get_calendar` function arguments before they are used.
	 *
	 * @since 6.8.0
	 *
	 * @param array $args {
	 *     Optional. Arguments for the `get_calendar` function.
	 *
	 *     @type bool   $initial   Whether to use initial calendar names. Default true.
	 *     @type bool   $display   Whether to display the calendar output. Default true.
	 *     @type string $post_type Optional. Post type. Default 'post'.
	 * }
	 * @return array The arguments for the `get_calendar` function.
	 */
	$args = apply_filters( 'get_calendar_args', wp_parse_args( $args, $defaults ) );

	if ( ! post_type_exists( $args['post_type'] ) ) {
		$args['post_type'] = 'post';
	}

	$w = 0;
	if ( isset( $_GET['w'] ) ) {
		$w = (int) $_GET['w'];
	}

	/*
	 * Normalize the cache key.
	 *
	 * The following ensures the same cache key is used for the same parameter
	 * and parameter equivalents. This prevents `post_type > post, initial > true`
	 * from generating a different key from the same values in the reverse order.
	 *
	 * `display` is excluded from the cache key as the cache contains the same
	 * HTML regardless of this function's need to echo or return the output.
	 *
	 * The global values contain data generated by the URL query string variables.
	 */
	$cache_args = $args;
	unset( $cache_args['display'] );

	$cache_args['globals'] = array(
		'm'        => $m,
		'monthnum' => $monthnum,
		'year'     => $year,
		'week'     => $w,
	);

	wp_recursive_ksort( $cache_args );
	$key   = md5( serialize( $cache_args ) );
	$cache = wp_cache_get( 'my_get_calendar', 'calendar' );

	if ( $cache && is_array( $cache ) && isset( $cache[ $key ] ) ) {
		/** This filter is documented in wp-includes/general-template.php */
		$output = apply_filters( 'my_get_calendar', $cache[ $key ], $args );

		if ( $args['display'] ) {
			echo $output;
			return;
		}

		return $output;
	}

	if ( ! is_array( $cache ) ) {
		$cache = array();
	}

	$post_type = $args['post_type'];

	// Quick check. If we have no posts at all, abort!
	if ( ! $posts ) {
		$gotsome = $wpdb->get_var(
			$wpdb->prepare(
				"SELECT 1 as test
				FROM $wpdb->posts
				WHERE ($custom_post_type OR post_type = %s)
				AND post_status = 'publish'
				LIMIT 1",
				$post_type
			)
		);

		if ( ! $gotsome ) {
			$cache[ $key ] = '';
			wp_cache_set( 'my_get_calendar', $cache, 'calendar' );
			return;
		}
	}

	// week_begins = 0 stands for Sunday.
	$week_begins = (int) get_option( 'start_of_week' );

	// Let's figure out when we are.
	if ( ! empty( $monthnum ) && ! empty( $year ) ) {
		$thismonth = (int) $monthnum;
		$thisyear  = (int) $year;
	} elseif ( ! empty( $w ) ) {
		// We need to get the month from MySQL.
		$thisyear = (int) substr( $m, 0, 4 );
		// It seems MySQL's weeks disagree with PHP's.
		$d         = ( ( $w - 1 ) * 7 ) + 6;
		$thismonth = (int) $wpdb->get_var(
			$wpdb->prepare(
				"SELECT DATE_FORMAT((DATE_ADD('%d0101', INTERVAL %d DAY) ), '%%m')",
				$thisyear,
				$d
			)
		);
	} elseif ( ! empty( $m ) ) {
		$thisyear = (int) substr( $m, 0, 4 );
		if ( strlen( $m ) < 6 ) {
			$thismonth = 1;
		} else {
			$thismonth = (int) substr( $m, 4, 2 );
		}
	} else {
		$thisyear  = (int) current_time( 'Y' );
		$thismonth = (int) current_time( 'm' );
	}

	$unixmonth = mktime( 0, 0, 0, $thismonth, 1, $thisyear );
	$last_day  = gmdate( 't', $unixmonth );

	// Get the next and previous month and year with at least one post.
	$previous = $wpdb->get_row(
		$wpdb->prepare(
			"SELECT MONTH(post_date) AS month, YEAR(post_date) AS year
			FROM $wpdb->posts
			WHERE post_date < '%d-%d-01'
			AND ($custom_post_type OR post_type = %s) AND post_status = 'publish'
			ORDER BY post_date DESC
			LIMIT 1",
			$thisyear,
			zeroise( $thismonth, 2 ),
			$post_type
		)
	);

	$next = $wpdb->get_row(
		$wpdb->prepare(
			"SELECT MONTH(post_date) AS month, YEAR(post_date) AS year
			FROM $wpdb->posts
			WHERE post_date > '%d-%d-%d 23:59:59'
			AND ($custom_post_type OR post_type = %s) AND post_status = 'publish'
			ORDER BY post_date ASC
			LIMIT 1",
			$thisyear,
			zeroise( $thismonth, 2 ),
			$last_day,
			$post_type
		)
	);

	/* translators: Calendar caption: 1: Month name, 2: 4-digit year. */
	$calendar_caption = _x( '%1$s %2$s', 'calendar caption' );
	$calendar_output  = '<table id="wp-calendar" class="wp-calendar-table">
	<caption>' . sprintf(
		$calendar_caption,
		$wp_locale->get_month( $thismonth ),
		gmdate( 'Y', $unixmonth )
	) . '</caption>
	<thead>
	<tr>';

	$myweek = array();

	for ( $wdcount = 0; $wdcount <= 6; $wdcount++ ) {
		$myweek[] = $wp_locale->get_weekday( ( $wdcount + $week_begins ) % 7 );
	}

	foreach ( $myweek as $wd ) {
		$day_name         = $args['initial'] ? $wp_locale->get_weekday_initial( $wd ) : $wp_locale->get_weekday_abbrev( $wd );
		$wd               = esc_attr( $wd );
		$calendar_output .= "\n\t\t<th scope=\"col\" aria-label=\"$wd\">$day_name</th>";
	}

	$calendar_output .= '
	</tr>
	</thead>
	<tbody>
	<tr>';

	$daywithpost = array();

	// Get days with posts.
	$dayswithposts = $wpdb->get_results(
		$wpdb->prepare(
			"SELECT DISTINCT DAYOFMONTH(post_date)
			FROM $wpdb->posts WHERE post_date >= '%d-%d-01 00:00:00'
			AND ($custom_post_type OR post_type = %s) AND post_status = 'publish'
			AND post_date <= '%d-%d-%d 23:59:59'",
			$thisyear,
			zeroise( $thismonth, 2 ),
			$post_type,
			$thisyear,
			zeroise( $thismonth, 2 ),
			$last_day
		),
		ARRAY_N
	);

	if ( $dayswithposts ) {
		foreach ( (array) $dayswithposts as $daywith ) {
			$daywithpost[] = (int) $daywith[0];
		}
	}

	// See how much we should pad in the beginning.
	$pad = calendar_week_mod( (int) gmdate( 'w', $unixmonth ) - $week_begins );
	if ( $pad > 0 ) {
		$calendar_output .= "\n\t\t" . '<td colspan="' . esc_attr( $pad ) . '" class="pad"> </td>';
	}

	$newrow      = false;
	$daysinmonth = (int) gmdate( 't', $unixmonth );

	for ( $day = 1; $day <= $daysinmonth; ++$day ) {
		if ( isset( $newrow ) && $newrow ) {
			$calendar_output .= "\n\t</tr>\n\t<tr>\n\t\t";
		}

		$newrow = false;

		if ( (int) current_time( 'j' ) === $day
			&& (int) current_time( 'm' ) === $thismonth
			&& (int) current_time( 'Y' ) === $thisyear
		) {
			$calendar_output .= '<td id="today">';
		} else {
			$calendar_output .= '<td>';
		}

		if ( in_array( $day, $daywithpost, true ) ) {
			// Any posts today?
			$date_format = gmdate( _x( 'F j, Y', 'daily archives date format' ), strtotime( "{$thisyear}-{$thismonth}-{$day}" ) );
			/* translators: Post calendar label. %s: Date. */
			$label            = sprintf( __( 'Posts published on %s' ), $date_format );
			$calendar_output .= sprintf(
				'<a href="%s" aria-label="%s">%s</a>',
				get_day_link( $thisyear, $thismonth, $day ),
				esc_attr( $label ),
				$day
			);
		} else {
			$calendar_output .= $day;
		}

		$calendar_output .= '</td>';

		if ( 6 === (int) calendar_week_mod( (int) gmdate( 'w', mktime( 0, 0, 0, $thismonth, $day, $thisyear ) ) - $week_begins ) ) {
			$newrow = true;
		}
	}

	$pad = 7 - calendar_week_mod( (int) gmdate( 'w', mktime( 0, 0, 0, $thismonth, $day, $thisyear ) ) - $week_begins );
	if ( 0 < $pad && $pad < 7 ) {
		$calendar_output .= "\n\t\t" . '<td class="pad" colspan="' . esc_attr( $pad ) . '"> </td>';
	}

	$calendar_output .= "\n\t</tr>\n\t</tbody>";

	$calendar_output .= "\n\t</table>";

	$calendar_output .= '<nav aria-label="' . __( 'Previous and next months' ) . '" class="wp-calendar-nav">';

	if ( $previous ) {
		$calendar_output .= "\n\t\t" . sprintf(
			'<span class="wp-calendar-nav-prev"><a href="%1$s">« %2$s</a></span>',
			get_month_link( $previous->year, $previous->month ),
			$wp_locale->get_month_abbrev( $wp_locale->get_month( $previous->month ) )
		);
	} else {
		$calendar_output .= "\n\t\t" . '<span class="wp-calendar-nav-prev"> </span>';
	}

	$calendar_output .= "\n\t\t" . '<span class="pad"> </span>';

	if ( $next ) {
		$calendar_output .= "\n\t\t" . sprintf(
			'<span class="wp-calendar-nav-next"><a href="%1$s">%2$s »</a></span>',
			get_month_link( $next->year, $next->month ),
			$wp_locale->get_month_abbrev( $wp_locale->get_month( $next->month ) )
		);
	} else {
		$calendar_output .= "\n\t\t" . '<span class="wp-calendar-nav-next"> </span>';
	}

	$calendar_output .= '
	</nav>';

	$cache[ $key ] = $calendar_output;
	wp_cache_set( 'my_get_calendar', $cache, 'calendar' );

	/**
	 * Filters the HTML calendar output.
	 *
	 * @since 3.0.0
	 * @since 6.8.0 Added the `$args` parameter.
	 *
	 * @param string $calendar_output HTML output of the calendar.
	 * @param array  $args {
	 *     Optional. Array of display arguments.
	 *
	 *     @type bool   $initial   Whether to use initial calendar names. Default true.
	 *     @type bool   $display   Whether to display the calendar output. Default true.
	 *     @type string $post_type Optional. Post type. Default 'post'.
	 * }
	 */
	$calendar_output = apply_filters( 'my_get_calendar', $calendar_output, $args );

	if ( $args['display'] ) {
		echo $calendar_output;
		return;
	}

	return $calendar_output;
}


/**
 * Plugin Name: Custom Calendar Widget
 */ 
add_action( 'widgets_init', 'register_custom_calendar_widget' );

function register_custom_calendar_widget() {
    register_widget( 'Custom_Calendar_Widget' );
}

class Custom_Calendar_Widget extends WP_Widget {

    public function __construct() {
        parent::__construct(
            'custom_calendar_widget', // Base ID
            __( 'カスタムカレンダー', 'text_domain' ), // Name
            array( 'description' => __( '複数のカスタム投稿タイプに対応する投稿カレンダー', 'text_domain' ), ) // Args
        );
    }

    public function widget( $args, $instance ) {
        echo $args['before_widget'];
        if ( ! empty( $instance['title'] ) ) {
            echo $args['before_title'] . apply_filters( 'widget_title', $instance['title'] ) . $args['after_title'];
        }
	?>
		<div id="calendar_wrap_custom" class="widget_calendar">
	<?php
		my_get_calendar();	// カスタムカレンダー表示
	?>
		</div>
	<?php
        echo $args['after_widget'];
    }

    public function form( $instance ) {
        $title = ! empty( $instance['title'] ) ? $instance['title'] : __( 'New title', 'text_domain' );
        ?>
        <p>
            <label for="<?php echo $this->get_field_id( 'title' ); ?>"><?php _e( 'Title:' ); ?></label>
            <input class="widefat" id="<?php echo $this->get_field_id( 'title' ); ?>" name="<?php echo $this->get_field_name( 'title' ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>">
        </p>
        <?php
    }

    public function update( $new_instance, $old_instance ) {
        $instance = array();
        $instance['title'] = ( ! empty( $new_instance['title'] ) ) ? strip_tags( $new_instance['title'] ) : '';
        return $instance;
    }
}

/**
 * カスタムカレンダーウィジェットのショートコード出力
 *
 * 呼び出し方法：[custom_calendar]
 */
function Custom_Calendar_Shortcode() {
	// ショートコード出力
	$cal = my_get_calendar(array('initial'=>true, 'display'=>false));
	return '<div id="shortcode_calendar_wrap_custom" class="widget_calendar">' . $cal . '</div>';

}
add_shortcode( 'custom_calendar', 'Custom_Calendar_Shortcode' );

<h1 class="widget-title">投稿カレンダー（ニュース）</h1>
<div id="calendar_wrap_custom" class="widget_calendar">

<?php
get_calendar( array( 'initial' => true, 'display' => true, 'post_type' => 'news'));
?>
</div>

/*
	カスタムカレンダーウィジェットの表示調整
*/
#calendar_wrap_custom {
	width: 90%;
	margin: auto;
}

/*--------------------------------------------------------
  ウィジェットデザイン設定
--------------------------------------------------------*/

/* カレンダー */
.widget_calendar div{
	padding-left: 20px;
	padding-right: 20px;
	padding-bottom: 20px;
}
.widget_calendar caption{
	font-weight: bold;
	padding-top: 5px;
	padding-bottom: 5px;
}
.widget_calendar table{
	/*color: #757575;*/	/* 標準文字色 */
	color: #808080;		/* カレンダー文字色 by Senri */
	width: 100%;
	border-collapse: collapse;
	border-top: none;
	border-right: none;
	border-bottom: none;
	border-left: none;
	table-layout: fixed;
}

.widget_calendar table td,
.widget_calendar table th{
	background-color: #121212;
	padding-top: 1px;
	padding-bottom: 1px;
	padding-right: 0;
	padding-left: 0;
	text-align: center;
	border-top: none;
	border-right: none;
	border-bottom: none;
	border-left: none;
}
.widget_calendar #today{
	background-color: #222222;
	border-radius: 5px;
}
.widget_calendar thead th,
.widget_calendar tbody td{
	border-top: solid 2px #222222;
	border-right: solid 2px #222222;
	border-bottom: solid 2px #222222;
	border-left: solid 2px #222222;
}
.widget_calendar tbody a{
	color: #000000;
	background-color: #CA9B33;
	padding-left: 3px;
	padding-right: 3px;
	border-radius: 5px;
}
.widget_calendar #prev{
	padding-top: 3px;
	padding-bottom: 3px;
	padding-left: 3px;
	text-align: left;
}
.widget_calendar #next{
	padding-top: 3px;
	padding-bottom: 3px;
	padding-right: 3px;
	text-align: right;
}