the-kenny braucht Hauptmensa sonst weint er.

gistfile1.js

var kenny = (function() {

    /**
     * uri
     *
     * has to be enhanced with this \d\d\d\d-\d\d\d\d.htm
     * @param string
     */
    var host = "http://studentenwerk.essen-duisburg.de/speiseplaene/kl_";

    var date = null;

    var showNextWeek = true;

    function getDay () {

        if (date === null) {
            date = new Date();
        }

        var day = date.getDate();
        if (day < 10) {
            return "0" + day;
        }

        return day;
    };

    function getMonth () {

        if (date === null) {
            console.log("month is leer");
            date = new Date();
        }

        var month = date.getMonth();
        month += 1;
        if (month < 10) {
            return "0" + month;
        }

        return month;
    }

    return {
        getUri: function () {

            date    = new Date();
            var day = date.getDay();

            if (showNextWeek) {
                if (day === 0) {
                    date = new Date(date.getTime() + 86400000);
                } else if (day === 6) {
                    date = new Date(date.getTime() + (86400000)*2);
                }
            }

            return host + getDay() + getMonth() + "-" + getDay() + getMonth() + ".htm";
        }
    };
})();

Use it with
document.location = kenny.getUri();
as bookmarklet.