Udhay Titus

var option = '';
for (var i=1;i<32;i++){
   option += '<option value="'+ i + '">' + i + '</option>';
}
$('#days').append(option);

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select id="days"></select>