try this
<html>
<head>
<script language="JavaScript">
function week() {
var n, day;
n = parseInt( document.formday.nday.value);
switch (n) {
case 1: day = "monday";
break;
case 2: day = "tuesday";
break;
case 3: day = "wednesday";
break;
case 4: day = "thursday";
break;
case 5: day = "friday";
break;
case 6: day = "saturday";
break;
case 7: day = "sunday";
break;
default: day = "invalid input";
break;
}
document.formday.txtno.value = day;
}
</script>
</head>
<body>
<form action="login.php" method="post" name="formday">
daynumber <input type="text" name="nday">
day <input type="text" name="txtno">
<input type="button" value="show" onclick="week()">
</form>
</body>
</html>