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