Get the date for the first advent.

get1stAdvent.php

<?php

/**
 * @param string $date - [YYYY-MM-DD]
 * @return int
 */
function getWeekday( string $date ):int {
  // 'w' 0 (for Sunday) through 6 (for Saturday)
  $result = (int) date('w', strtotime($date));

  return $result;
}

/**
 * @param string $christmasEve - [YYYY-MM-DD]
 * @param string $format - 'd.m.Y'
 * @return string
 */
function get1stAdvent( string $christmasEve, string $format = 'd.m.Y' ):string {
  $datetime = new DateTime($christmasEve);
  $diff = getWeekday($christmasEve) + 21;
  $datetime->modify('-' . $diff . ' day');

  return $datetime->format($format);
}


/**
 * Demo - get the 1st advent for the next 10 years
 */
echo '<pre>';
for ($i = 0; $i <= 10; $i++):
  $year = date('Y') + $i;
  echo get1stAdvent($year . '-12-24', 'd.m.Y');
  echo '<br>';
endfor;
echo '</pre>';