These docs are for an old release. Info on upgrading to version 2/3
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dayClick

Triggered when the user clicks on a day.

function( date, allDay, jsEvent, view ) { }

date holds a Date object for the current day. If user has clicked on a slot in the agendaWeek or agendaDay views, date will also have its time set.

allDay will be set to false if the user has clicked on a slot in the agendaWeek or agendaDay views. Otherwise, it will be true.

jsEvent holds the native JavaScript event with low-level information such as click coordinates.

view is set to the current View Object.

Within the callback function, this is set to the <td> of the clicked day.

Here is an example that demonstrates all of these variables:

$('#calendar').fullCalendar({
  dayClick: function(date, allDay, jsEvent, view) {

    if (allDay) {
      alert('Clicked on the entire day: ' + date);
    }else{
      alert('Clicked on the slot: ' + date);
    }

    alert('Coordinates: ' + jsEvent.pageX + ',' + jsEvent.pageY);

    alert('Current view: ' + view.name);

    // change the day's background color just for fun
    $(this).css('background-color', 'red');

  }
});

The allDay parameter has only been available since version 1.4.