PHP - Php Highlight Dates Between Two Dates On Calender

I am trying to create a simple calendar where a user can add/remove their availability date. This part works so far. Now what I am having trouble with is highlighting the already selected dates that are fetched from a MySQL database.

Here's my code.

// HTML
<div id="datetimepicker1"></div>

// Jquery/Ajax

<script>
$(document).ready(function () {

  $('#datetimepicker1').datepicker({
    dateFormat: "yy-mm-dd",
    multidate: true,
    onSelect: function () {
        var getDate = $("#datetimepicker1").val();

        $.ajax({
             type: "POST", //or GET. Whichever floats your boat.
             url: "snippets/adapter-set.php",
             data: { date: getDate },
             success: function(data) {
              // alert(data);
             },
             error: function() {
                 alert("Error.");
             }
        });
    }
  });
});
</script>

// PHP
// adapter-set.php

$post_date = $_POST['date'];

$find_query = $db->prepare("SELECT user_id FROM user_dates WHERE date_available = :date_available");
$find_query->bindParam(':date_available', $post_date);
$find_query->execute();
$result_find = $find_query->fetchAll(PDO::FETCH_ASSOC);
if(count($result_find) > 0) {
  foreach($result_find as $row) {
    $user_id =	$row['user_id'];
  }
  $delete_query = $db->prepare("DELETE FROM user_dates WHERE user_id = :user_id");
  $delete_query->bindParam(':user_id', $user_id);
  $delete_query->execute();
  $result_delete = $delete_query->execute();
  if($result_delete == false) {
   echo 'delete false';
  } else {
   echo 'delete success';
  }
} else {
  $insert_query = $db->prepare("INSERT INTO user_dates(date_available) VALUES(:date_available)");
  $insert_query->bindParam(':date_available', $post_date);
  $result_insert = $insert_query->execute();
  if($result_insert == false) {
   echo 'insert false';
  } else {
   echo 'insert success';
  }

}

The above code works fine. It inserts and deletes a date row in MySQL database table based on a click.

Now what I would like to do is to highlight all the "available" dates that are already inserted into the database; so that the user knows which dates he has already selected. This is my code for that. But it doesn't seem to be working. No errors. It's just not highlighting the inserted dates. Can you tell me what I'm doing wrong?

// JQUERY
<script>
    $(document).ready(function() {

        $.post('snippets/adapter-fetch.php', {}, function(data){
            $("#datetimepicker1").datepicker({
              datesEnabled : data.datesEnabled
            });
        }, 'json');

    });
</script>

// PHP
// adapter-fetch.php

$global_user_id = 5;

$find_query = $db->prepare("SELECT date_available FROM user_dates WHERE user_id = :user_id");
$find_query->bindParam(':user_id', $global_user_id);
$find_query->execute();
$result_find = $find_query->fetchAll(PDO::FETCH_ASSOC);
if(count($result_find) > 0) {
  foreach($result_find as $row) {
    $date_available =	$row['date_available'];

    echo $date_available;
  }
} else {
  echo 'not dates available';
}