FullCalendar function to "grey out" days before the current date (it does this by applying the 'fc-other-month' style, which is what is used for dates that appear in another month). Uses the moment.js library to (more easily) manipulate date objects.

gistfile1.js

  $('#calendar').fullCalendar({
      // Other Calendar Options go here
      viewDisplay: function(view){
        $('.fc-day').filter(
          function(index){
          return moment( $(this).data('date') ).isBefore(moment(),'day') 
        }).addClass('fc-other-month');
      }      
    })