The name of your free format variable(s) needs to be consistent with the names used in the HTML and JavaScript. Sticking with the names "_Unix," "_Year", "_Month", "_Day", and/or "_ReadableString" might be easiest to make sure data is saved.
To restrict the calendar to today and earlier, you can add "maxDate" to the code like this:
$('#datePicker').datepicker({
maxDate: 'today',
onClose: function(){
...
To skip to a different question when Labor Day is selected, you'll need to add a skip to your free format question that skips when _Month = 9 and _Day = 2, 3, or 4:
fecha_Month = 9 And (fecha_Day = 2 Or fecha_Day = 3 Or fecha_Day = 4)