foo.js

jQuery('main').on('click', '#terminstandorte select option, .nextMonth, .prevMonth', function(){
    var request = "openDates";

    if (jQuery(this).get(0) == jQuery('.nextMonth').get(0) || jQuery(this).get(0) == jQuery('.prevMonth').get(0)) {
        jQuery('#kalender').load('?' + jQuery(this).attr('value') + ' #thisMonth', getFreeDates);
    }

    function getFreeDates() {
        var werk = +jQuery('#terminstandorte select option:selected').attr('value');
        var firstDay = jQuery('.calDates .noDate:first').attr('id');
        var lastDay = jQuery('.calDates .noDate:last').attr('id');

        // RegExp‽
        firstDay = firstDay.split("-");
        firstDay = firstDay[1]+","+firstDay[2]+","+firstDay[0]
        firstDay = parseInt(new Date(firstDay).getTime(), 10);
        lastDay = lastDay.split("-");
        lastDay = lastDay[1]+","+lastDay[2]+","+lastDay[0]
        lastDay = parseInt(new Date(lastDay).getTime(), 10);


        //Hier werden alle freien Termine für den aktuellen Monat angezeigt
        jQuery.ajax({
            url: CENSORED,
            type: "GET",
            data: {
                request: request,
                werk: werk,
                firstDay: firstDay,
                lastDay: lastDay
            },
            contentType: 'application/json; charset=utf-8',
            beforeSend: function () {
                // console.log(firstDay, lastDay);
                var today = new Date(firstDay);
                console.log(today.getMonth()+1);
            },
            success: function (data) {
                var freeDates = JSON.parse(data);
                freeDates.forEach(function (date) {
                    doSomething();
                });
            },
            error: function () {
                alert('Etwas lief schief');
            }
        });
    }
});