土日祝日を除いた平日の日付リストを日数指定で取得します。
祝日判定には以前作成した祝日取得クラスを利用してみました。

weekdays.php

<?php
require_once('holiday_class.php');
class Weekdays {
  private $holiday;
  private $startDate;

  public function __construct() {
    $this->holiday = new Holiday;
    $this->startDate = new DateTimeImmutable;
  }

  public function setStartDate($ymd = 'now') {
    $this->startDate = new DateTimeImmutable($ymd);
  }

  private function isHoliday($date) {
    [$y, $n, $j] = explode(',', $date->format('Y,n,j'));
    return isset($this->holiday->getHolidayOfYear($y)[$n][$j]);
  }

  public function getDateList($days = 1) {
    $days = max(1, $days);
    $currentDate = new DateTime;
    $currentDate->setTimestamp($this->startDate->getTimestamp());
    $result = [];

    do {
      while(strpos('06', $currentDate->format('w')) !== false ||
            $this->isHoliday($currentDate)) {
        $currentDate->modify('+1 day');
      }
      $result[] = $currentDate->format('Y-m-d');
      $currentDate->modify('+1 day');
    } while(--$days > 0);

    return $result;
  }
}

example####

// example
$weekdays = new Weekdays;

$date_list = $weekdays->getDateList(7);
print_r($date_list);
/*
  Array
  (
      [0] => 2021-09-17
      [1] => 2021-09-21
      [2] => 2021-09-22
      [3] => 2021-09-24
      [4] => 2021-09-27
      [5] => 2021-09-28
      [6] => 2021-09-29
  )
*/

foreach($date_list as $k => $v) {
  printf("%d: %s (%s)\n",
    $k, $v, ['日','月','火','水','木','金','土'][date('w', strtotime($v))]
  );
}
/*
  0: 2021-09-17 (金)
  1: 2021-09-21 (火)
  2: 2021-09-22 (水)
  3: 2021-09-24 (金)
  4: 2021-09-27 (月)
  5: 2021-09-28 (火)
  6: 2021-09-29 (水)
*/

$weekdays->setStartDate('2021-12-20');
$date_list = $weekdays->getDateList(20);
print_r($date_list);
/*
  Array
  (
      [0] => 2021-12-20
      [1] => 2021-12-21
      [2] => 2021-12-22
      [3] => 2021-12-23
      [4] => 2021-12-24
      [5] => 2021-12-27
      [6] => 2021-12-28
      [7] => 2021-12-29
      [8] => 2021-12-30
      [9] => 2021-12-31
      [10] => 2022-01-03
      [11] => 2022-01-04
      [12] => 2022-01-05
      [13] => 2022-01-06
      [14] => 2022-01-07
      [15] => 2022-01-11
      [16] => 2022-01-12
      [17] => 2022-01-13
      [18] => 2022-01-14
      [19] => 2022-01-17
  )
*/