Disable particular date in datepicker in php

Question

I want to disable the particular date in datepicker.. When user selects particular date dropdown displays with details.. I want to disable 2021-12-25 date in datepicker.. I added the code to disable date still it doesnt work Here is the code

 <script type="text/javascript">
        var dates = ["2021-12-25"];
    function DisableDates(date) {
        var string = jQuery.datepicker.formatDate('yy-mm-dd', date);
        return [dates.indexOf(string) == -1];
    }
    
    
    </script>

    <script type="text/javascript">
    $(function() {
      var date = new Date();
      var dayNo = date.getDay();
    
      var d = ['sun', 'mon', 'tue', 'wed', 'thu', 'fri', 'sat'];
      var event = ['6.00 am : English', '7.00 am : Kannada', '8.00 am: Tamil'];
      var event1 = ['7.00 am : Kannada', '9.00 am: English', '11.00 am: Tamil'];
 
     var custDate = ['2021-12-24'];
   
  
   
    $("#datepicker").datepicker({
        changeMonth: true,
        dateFormat: 'yy-mm-dd',
         minDate: 0,
        onSelect: function(dateText, inst) {
          var today = new Date(dateText);
          var a = (d[today.getDay()]);
          $('#slDay').val(d[today.getDay()]);
          var html = '';
          $('#slDay').html('');
          if (custDate.includes(dateText)) {
            $.each(event3, function(index, value) {
              html += '<option value="' + value + '">' + value + '</option>'
            });
          } 
          
         else {
            $.each(event, function(index, value) {
              html += '<option value="' + value + '">' + value + '</option>'
            });
          }
          $('#slDay').append(html);
        }
      });
    });
    
    </script>