if (typeof options.isInvalidDate === 'function')
            this.isInvalidDate = options.isInvalidDate;

        if (typeof options.isCustomDate === 'function')
            this.isCustomDate = options.isCustomDate;

        if (typeof options.alwaysShowCalendars === 'boolean')
            this.alwaysShowCalendars = options.alwaysShowCalendars;

        // update day names order to firstDay