Hello @kartik,
In javascript, make an ajax function,
function myAjax() {
      $.ajax({
           type: "POST",
           url: 'your_url/ajax.php',
           data:{action:'call_this'},
           success:function(html) {
             alert(html);
           }
      });
 }
Then call from html,
<a href="" onclick="myAjax()" class="deletebtn">Delete</a>
And in your ajax.php,
if($_POST['action'] == 'call_this') {
  // call removeday() here
}
Hope it works!