in fullcalendar when clicking on an event with an associated url

You can use the eventClick callback in fullcalendar

$('#calendar').fullCalendar({
events: [
    {
        title: 'My Event',
        start: '2010-01-01',
        url: 'http://stackoverflow.com/'
    }
    // other events here
],
eventClick: function(event) {
    if (event.url) {
        window.open(event.url, "_blank");
        return false;
    }
}
});

If using V4 of FullCalendar, try this instead:

  eventClick: function(event) {
    if (event.event.url) {
      event.jsEvent.preventDefault()
      window.open(event.event.url, "_blank");
    }
  }