<script>
    var mygroup = new Array();

    mygroup[0] = new Array();
    mygroup[0][0] = new Option("Day", 0);

    mygroup[1] = new Array();
    mygroup[1][0] = new Option("1");
    mygroup[1][1] = new Option("2");
    mygroup[1][2] = new Option("3");

    mygroup[2] = new Array();
    mygroup[2][0] = new Option("1");
    mygroup[2][1] = new Option("2");
    mygroup[2][2] = new Option("3");

    mygroup[3] = new Array();
    mygroup[3][0] = new Option("1");
    mygroup[3][1] = new Option("2");
    mygroup[3][2] = new Option("3");
		
        function ResetSubSelect(form, subSelect)
        {
            var e = form.elements[subSelect];
            for (var i = 0; i < e.options.length; ++i) {
                e.options[i] = null;
            }
        }

        function DisableSubSelect(elem)
        {
            elem.disabled = 1;
        }

        function EnableSubSelect(elem)
        {
            elem.disabled = 0;
        }

        function ShowSubSelect(elem, subSelect)
        {
            ResetSubSelect(elem.form, subSelect);

            var i = elem.options[elem.selectedIndex].value;
            var s = elem.form.elements[subSelect];

            for (var k = 0; k < mygroup[i].length; k++) {
                s.options[k] = mygroup[i][k];
            }

            if (i == 0) {
                DisableSubSelect(s);
            } else {
                EnableSubSelect(s);
            }
        }

        function InitSubSelect()
        {
            ShowSubSelect(document.forms["myform"].elements["select_month"], "select_day");
        }
    </script>

 <select name='select_month' onChange='ShowSubSelect(this,'select_day')'>