jQuery addClass example

Change class name on click in jQuery

HTML

<head>
<script src="https://unpkg.com/jquery@3.4.1/dist/jquery.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.bundle.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.8.2/moment.min.js"></script>

<link href='https://unpkg.com/@fullcalendar/core@4.3.1/main.min.css' rel='stylesheet' />
<link href='https://unpkg.com/@fullcalendar/daygrid@4.3.0/main.min.css' rel='stylesheet' />
<link href='https://unpkg.com/@fullcalendar/timegrid@4.3.0/main.min.css' rel='stylesheet' />
<link href='https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css' rel='stylesheet' />
<script src='https://unpkg.com/@fullcalendar/core@4.3.1/main.min.js'></script>
<script src='https://unpkg.com/@fullcalendar/interaction@4.3.0/main.min.js'></script>
<script src='https://unpkg.com/@fullcalendar/daygrid@4.3.0/main.min.js'></script>
<script src='https://unpkg.com/@fullcalendar/timegrid@4.3.0/main.min.js'></script>
</head>

<body>
    <div id='calendar'></div>
</body>

<div id="fullCalModal" class="modal fade">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-header">
                <h4 id="modalTitle" class="modal-title"></h4>
            </div>
            <div id="modalBody" class="modal-body"></div>
        </div>
    </div>
    Test
</div>

<script>

    function loadCalendar() {
        var calendarEl = document.getElementById('calendar');
        var calendar = new FullCalendar.Calendar(calendarEl, {
            locale: 'pl',
            selectable: true,
            plugins: ['interaction', 'dayGrid'],
            firstDay : 1,
            header: {
                left: 'today',
                center: 'title',
                right: 'prev, next',
            },
            events: [
            	{
              title: 'All Day Event',
              description: 'www',
              start: '2019-09-03'
            	},
            ],

            eventClick:...