day-of-the-month.php

<?php
/**
 * Plugin Name: Day-of-the-Month Filter
 * Description: Adds a day-of-the-month filter to the posts list table in wp-admin.
 * Version: 1.0.0
 * Author: Gary Jones
 */

// Exit if accessed directly.
if ( ! defined( 'ABSPATH' ) ) {
	exit;
}

add_action( 'restrict_manage_posts', 'gmj_day_of_month_filter' );
/**
 * Add custom day-of-the-month filter to the posts list table.
 */
function gmj_day_of_month_filter() {
	global $wp_locale;

	// Ensure this is the posts list table.
	if ( ! function_exists( 'get_current_screen' ) || 'edit-post' !== get_current_screen()->id ) {
		return;
	}

	// Get selected day from query string.
	$selected_day = isset( $_GET['day'] ) ? intval( $_GET['day'] ) : '';

	// Output day-of-the-month filter HTML.
	?>
	<label class="screen-reader-text" for="day_of_month"><?php esc_html_e( 'Day', 'day-of-the-month-filter' ); ?></label>
	<select name="day" id="day_of_month">
		<option value=""><?php esc_html_e( 'All Days', 'day-of-the-month-filter' ); ?></option>
		<?php
		// Generate options for all 31 days.
		for ( $day = 1; $day <= 31; $day++ ) {
			printf(
				'<option value="%s" %s>%s</option>',
				$day,
				selected( $day, $selected_day, false ),
				$day
			);
		}
		?>
	</select>
	<?php
}