How to call events on clicking prev and next button?

The best way is:

viewRender: function(view, element) {
  var b = $('#calendar').fullCalendar('getDate');
  alert(b.format('L'));
},

This worked for me:

$('body').on('click', 'button.fc-prev-button', function() {
  //do something
});

$('body').on('click', 'button.fc-next-button', function() {
  //do something
});

You couldsimply attach an event to the button:

$('.fc-button-prev span').click(function(){
   alert('prev is clicked, do something');
});

$('.fc-button-next span').click(function(){
   alert('nextis clicked, do something');
});