WordPress datepicker adjustable filtered days. Example: https://jsfiddle.net/icetee/by9aa4pt/

picker.js

// @icetee - https://gist.github.com/icetee/946683aca3f0149aba5c56d14ded2897
(function($, opt) {
  var picker = function(opt) {
    var self = this;
    self.opt = opt;

    var $picker = $(self.opt.selector);
    if (!$picker) return;

    var attr = $.extend({
      beforeShowDay: function(date) {
        if (self.opt.showDays.indexOf(date.getDay()) > -1) {
          return [true, "", self.opt.available];
        }
        return [false, "", self.opt.unavailable];
      }
    }, self.opt.datepicker || {});

    $picker.datepicker(attr);
  };

  picker(opt);
})(jQuery, wmDatePickerOptions);

wm_datepicker.php

/*
 * Usage: [wm_datepicker name="datetime-735" class="wmdatepicker" showdays="1, 2" available="Available" unavailable="unAvailable" showdays="1, 2" mindate="0" maxdate="1M" dateformat="yy.mm.dd"]
 * @icetee - https://gist.github.com/icetee/946683aca3f0149aba5c56d14ded2897
 */
function wm_datepicker($atts) {
  $atts = shortcode_atts([
    'name' => 'datetime-' . rand(1, 99999999),
    'type' => 'text',
    'value' => '',
    'class' => 'wmdatepicker',
    'selector' => '.wmdatepicker',
    'mindate' => -0,
    'maxdate' => '1M',
    'dateformat' => 'yy.mm.dd',
    'showdays' => [4, 5, 6],
    'available' => 'Available',
    'unavailable' => 'unAvailable',
  ], $atts);

  $pickerOptions = json_encode([
    'selector' => $atts['selector'],
    'datepicker' => [
      'minDate' => $atts['mindate'],
      'maxDate' => $atts['maxdate'],
      'dateFormat' => $atts['dateformat'],
    ],
    'showDays' => array_map('intval', explode(', ', $atts['showdays'])),
    'available' => $atts['available'],
    'unavailable' => $atts['unavailable'],
  ]);

  echo '<script type="text/javascript">var wmDatePickerOptions='.$pickerOptions.';</script>';

  return '<input id="'. $atts["id"] .'" name="'. $atts["name"] .'" type="'. $atts["type"] .'" value="'. $atts["value"] .'" class="'. $atts["class"] .'" />';
}
add_shortcode('wm_datepicker', 'wm_datepicker');

function check_shortcode_existence() {
  global $post;
  if (is_a($post, 'WP_Post') && has_shortcode($post->post_content, 'wm_datepicker') && !is_admin()) {
    wp_register_script('wm_datepicker', get_theme_file_uri('/assets/js/picker.js', __FILE__), array('jquery', 'jquery-ui-datepicker'), '1.0', TRUE);
    wp_enqueue_script('wm_datepicker');

    $wp_scripts = wp_scripts();
    wp_enqueue_style(
      'plugin_name-admin-ui-css',
      'http://ajax.googleapis.com/ajax/libs/jqueryui/' . $wp_scripts->registered['jquery-ui-core']->ver . '/themes/smoothness/jquery-ui.css',
      FALSE,
      PLUGIN_VERSION,
      FALSE
    );
  }
}
add_action('wp_enqueue_scripts', 'check_shortcode_existence');