How do I disable sundays in the datepicker?

Fixed by changing alg-datepicker.js to:

/**
 * alg-datepicker.
 *
 * @version 1.0.0
 * @since   1.0.0
 */
function disableSUNDAYS(date) {
	var day = date.getDay();
	return [(day != 0), ''];
}
jQuery(document).ready(function() {
	jQuery("input[display='date']").each( function () {
		jQuery(this).datepicker({
			dateFormat : jQuery(this).attr("dateformat"),
			minDate : jQuery(this).attr("mindate"),
			maxDate : jQuery(this).attr("maxdate"),
			firstDay : jQuery(this).attr("firstday"),
			changeYear: jQuery(this).attr("changeyear"),
			yearRange: jQuery(this).attr("yearrange"),
			beforeShowDay: disableSUNDAYS
		});
	});
});