Ну не пустой же value.
<form name="add" method="post" action="file.php" onSubmit = "return check_valid(this)">
<select name="day">
<option value="день">день</option>
<input type="submit"/>
</form>
<script type="text/javascript">
function check_valid (form) {
alert (form.day.value);
}
</script>