JSFiddle - React, Tailwind, and code Playground

HTML

<link rel="stylesheet" href="http://davidwalsh.name/dw-content/jquery-ui-css/custom-theme/jquery-ui-1.7.2.custom.css">
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<script src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.7.2/jquery-ui.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.24.0/moment.min.js"></script>
<input type="text" name="date" id="date" readonly="readonly" size="12" />

JavaScript

var availableDates = ["02-01-2020","03-01-2020","04-01-2020","05-01-2020","06-01-2020"];

function available(date) {
  dayCustom = moment(date).format('DD-MM-YYYY');
  var valid = availableDates.includes(dayCustom)?false:true;/* si existe en el arreglo false */
  var _tooltip = valid ? '' : 'YA SELECCIONADO';
  return [valid, "",_tooltip];
}

$('#date').datepicker({ beforeShowDay: available });