Is there any (easy) way to set the jQuery UI Datepicker to disallow selection of specific, predetermined days?

I was able to get this approach working, however, it produces a null error which prevents it from displaying in IE.

'natDays[...].0' is null or not an object

Thanks in advance!

UPDATE: Might help if I included some code, right? Anyway, most of this was taken straight from the aforementioned thread:

natDays = [
	 	[7, 23], [7, 24], [8, 13], [8, 14], 
	];

	function nationalDays(date) {
	 	for (i = 0; i < natDays.length; i++) {
	 		if (date.getMonth() == natDays[i][0] - 1
	 		&& date.getDate() == natDays[i][1]) {
	 			return [false, natDays[i][2] + '_day'];
	 		}
	 	}
		return [true, ''];
	}

	function noWeekendsOrHolidays(date) {
	 	var noWeekend = $.datepicker.noWeekends(date);
	 	if (noWeekend[0]) {
	 		return nationalDays(date);
	 	} else {
	 		return noWeekend;
	 	}
	}


	$(function() { 
		$("#datepicker").datepicker({
			inline: true,
			minDate: new Date(2009, 6, 6), 
			maxDate: new Date(2009, 7, 14), 
			numberOfMonths: 2, 
			hideIfNoPrevNext: true,
			beforeShowDay: $.datepicker.noWeekends,
			altField: '#alternate',
		}); 
	});

Thanks again!