JSFiddle - React, Tailwind, and code Playground

by grant

HTML

<!DOCTYPE html>
<html>
<head>

<link href="//cdnjs.cloudflare.com/ajax/libs/fullcalendar/2.1.1/fullcalendar.min.css" rel="stylesheet" type="text/css" />
  
<script src="//code.jquery.com/jquery-2.1.1.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.8.3/moment.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/fullcalendar/2.1.1/fullcalendar.min.js"></script>
  
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>
  <div id='calendar'></div>
  
  <script>
   
  </script>
</body>
</html>

CSS

.fc-state-highlight{
  background:red
}

JavaScript

$(document).ready(function() {
      $('#calendar').fullCalendar({});

        var dates = ["2022-01-20", "2022-01-21"];

            
        $('.fc-day').each(function () {
            var thisdate = $(this).attr('data-date');
            var td = $(this).closest('td');

            if ($.inArray($(this).attr('data-date'), dates) !== -1) {
                td.addClass('fc-state-highlight');
            } else {
                td.removeClass('fc-state-highlight');
            }
        });
      
    });