Filtering by Date

I am trying to filter a report based on the month from a select box.
The query for the report

$sql = <<<EOT
    SELECT DATE_FORMAT(tbl_session.session_date,"%W, %M, %d, %Y") AS DATE, 
   TIME_FORMAT(CONCAT(tbl_session.time_slot_id, ":00:00"), "%h:%i %p") AS TIME,
   tbl_session.session_type AS TYPE, tbl_therapist.therapist_name as THERAPIST, tbl_session.session_status AS STATUS
    FROM tbl_session
    INNER JOIN tbl_therapist
    ON tbl_therapist.therapist_id=tbl_session.therapist_id
   WHERE tbl_session.client_id=$client_id
AND tbl_session.session_date = (NOT SURE WHAT GOES HERE)
    ORDER BY session_date DESC;
EOT;
$result = $db->query($sql);

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The selectbox

<form action="monthselect.php">
  <label for="monthly">Select a Month</label>
 
  <select name="monthly" id="monthly">
     <option value="All">Show All</option>
    <option value="Jan">January</option>
    <option value="Feb">February</option>
    <option value="Mar">March</option>
    <option value="Apr">April</option>
    <option value="May">May</option>
    <option value="Jun">June</option>
    <option value="Jul">July</option>
    <option value="Aug">August</option>
    <option value="Sept">September</option>
    <option value="Oct">October</option>
    <option value="Nov">November</option>
    <option value="Dec">December</option>
  </select>
   <input type="submit" value="Submit"> 
</form>

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I know I need to add an Ajax script and populate the monthselect.php file.  Not sure how.

<script>
      $('select[name=monthly]').on('change', function() {

    let selectedValue = $(this).val()
    $.ajax({
        url : 'monthselect.php',
        data : { client : selectedValue },
    }).done(function(response) {
        console.log(response);
      $('#results').html(response);
    })

})
      </script>

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