I use calendar.js and I want it to capture the dayclick event but it does not work for me

$(function() {
// page is now ready, initialize the calendar...
$('#calendar').fullCalendar({
    selectable: true,
    header: {
      left: 'prev,next today',
      center: 'title',
      right: 'month,agendaWeek,agendaDay'
    },
    dayClick: function(date) {
        alert('clicked ' + date.format());
      },
      select: function(startDate, endDate) {
        alert('selected ' + startDate.format() + ' to ' + endDate.format());
      }
    // weekNumbers: true,
    // eventLimit: true, // allow "more" link when too many events
    // events: 'https://fullcalendar.io/demo-events.json'
  });
 });