Returns the end of the last visible day. Note that in most cases this date is different from the last day of the last month. You can use this method (and visibleStart() method) to get the visible range and load the free/busy data when the view is changed.
DayPilot.Navigator.visibleEnd();
var nav = new DayPilot.Navigator("nav");
// ...
nav.init();
// ...
var start = nav.visibleEnd();