document.addEventListener('DOMContentLoaded', function() {
 var calendarEl = document.getElementById('calendar');
 var calendar = new FullCalendar.Calendar(calendarEl, {
  plugins: [ 'dayGrid' ],
  defaultView: 'dayGridMonth',
  eventRender: function(info) {
   var ntoday = moment().format('YYYYMMDD');
   var eventStart = moment( info.event.start ).format('YYYYMMDD');
   info.el.setAttribute("title", info.event.extendedProps.description);
   info.el.setAttribute("data-toggle", "tooltip");
   if (eventStart < ntoday){
    info.el.classList.add("fc-past-event");
   } else if (eventStart == ntoday){
      info.el.classList.add("fc-current-event");
   } else {
      info.el.classList.add("fc-future-event");
    }
  },

  events: [
    {
     title: 'All Day Event',
     description: 'description for All Day Event',
     start: '2020-01-01'
   }
   // more events here
  ]
 });
 calendar.render();
});

<script src="plugins/fullcalendar/core-4.3.1/main.min.js"></script>
<script src="plugins/fullcalendar/daygrid-4.3.0/main.min.js"></script>
<script src="js/calendar.js"></script>