■ヘッダ部分↓↓
<SCRIPT LANGUAGE="JavaScript">
<!--
function dropdown()
{
var nowdate = new Date();
var mon = nowdate.getMonth() + 1; // 月
var date = nowdate.getDate(); // 日
document.write("<option value=""></option>");
if (mon != 1)
document.write("<option value='1'>1</option>");
else if (date < 21)
document.write("<option value='1'>1</option>");
if (mon != 2)
document.write("<option value='2'>2</option>");
else if (date < 21)
document.write("<option value='2'>2</option>");
if (mon != 3)
document.write("<option value='3'>3</option>");
else if (date < 21)
document.write("<option value='3'>3</option>");