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
});
});
});