JavaScript - Ayuda con un codigo

<script>

// Get the element with id="defaultOpen" and click on it

// document.getElementById("defaultOpen").click();

var d = new Date();

  var weekday = new Array(7);

  weekday[0] =  "Domingo";

  weekday[1] = "Lunes";

  weekday[2] = "Martes";

  weekday[3] = "Miercoles";

  weekday[4] = "Jueves";

  weekday[5] = "Viernes";

  weekday[6] = "Sabado";

  var n = weekday[d.getDay()];

  document.getElementById(n+"Day").click();

</script>

<div class="tab">

  <button class="tablinks" id="LunesDay" onclick="openCity(event, 'Lunes')">Lunes</button>

  <button class="tablinks" id="MartesDay" onclick="openCity(event, 'Martes')">Martes</button>

  <button class="tablinks" id="MiercolesDay" onclick="openCity(event, 'Miércoles')">Miércoles</button>

  <button class="tablinks" id="JuevesDay" onclick="openCity(event, 'Jueves')">Jueves</button>

  <button class="tablinks" id="ViernesDay" onclick="openCity(event, 'Viernes')">Viernes</button>

  <button class="tablinks" id="SábadoDay" onclick="openCity(event, 'Sábado')">Sábado</button>

  <button class="tablinks" id="DomingoDay" onclick="openCity(event, 'Domingo')">Domingo</button>

</div>

<script src="https://code.jquery.com/jquery-3.2.1.min.js"></script>

<script type="text/javascript">

  $(document).ready(function(){

    var w=$(window).width();

    if( /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(navigator.userAgent) ) {

      $('.tablinks').each(function(index,el){

        var d=el.textContent;

        el.textContent=d.substring(0,1);

      });

    }

  });

</script>