Allow certain days to be disabled in Date field

I created a Date element, selected "Use date picker"

I then added this on page load:

        $("#ui-datepicker-div").datepicker("destroy");
        $("#ui-datepicker-div").datepicker({
          beforeShowDay: function(date) {
            var day = date.getDay();
            return [(day != 0), ''];
          }
        });
        $("#ui-datepicker-div").datepicker("refresh");

But nothing happens.

If I run the code in console, the window updates and disables Sundays. But when I click out and click again, it is back to normal. Is there something I am missing here?

Changing status for visibility. Sorry if this breaks any rules.