Anyway, show us the link (I assume you mean an <a href="..."> link) that you want to change.
And when you say you want it to show
/tagged/DAY-10/ do you mean that the
10 is just the day of the month?
Actually, if that's all you want, it's easy:
Code:
<html>
<body>
<a id="DAYLINK" href="/tagged/DAY-1/"> Today's link </a>
<script type="text/javascript">
var now = new Date();
document.getElementById("DAYLINK").href = "/tagged/DAY-" + now.getDate() + "/";
</script>
</body>
</html>