<div class="tab" style="margin-top: 1.6%;">
  <button class="tablinks" onclick="openCity(event, 'Sun0')">Sun</button>
  <button class="tablinks" onclick="openCity(event, 'Mon0')">Mon</button>
  <button class="tablinks" onclick="openCity(event, 'Tue0')">Tue</button>
  <button class="tablinks" onclick="openCity(event, 'Wed0')">Wed</button>
  <button class="tablinks" onclick="openCity(event, 'Thu0')">Thu</button>
  <button class="tablinks" onclick="openCity(event, 'Fri0')">Fri</button>

document.getElementsByClassName('tablinks').click()
function openCity(evt, cityName) {
  var i, tabcontent, tablinks;
  tabcontent = document.getElementsByClassName("tabcontent");
  for (i = 0; i < tabcontent.length; i++) {
    tabcontent[i].style.display = "none";
  }
  tablinks = document.getElementsByClassName("tablinks");
  for (i = 0; i < tablinks.length; i++) {
    tablinks[i].className = tablinks[i].className.replace(" active", "");
  }
  document.getElementById(cityName).style.display = "block";
  evt.currentTarget.className += " active";
}

<div id="Sun0" class="tabcontent">

<!--Sunday-->
<div style="overflow-x:auto;">
  <p> Contents of Sun0 </p>
</div>

<div id="Mon0" class="tabcontent">

<!--Monday-->
<div style="overflow-x:auto;">
 <p> Contents of Sun0 </p>
</div>

<div id="Tue0" class="tabcontent">

<!--Tueday-->
<div style="overflow-x:auto;">
 <p> Contents of Sun0 </p>
</div>

<div id="Wed0" class="tabcontent">

<!--Wenesday-->
<div style="overflow-x:auto;">
 <p> Contents of Sun0 </p>
</div>

<div id="Thu0" class="tabcontent">

 <!--Thursday-->
<div style="overflow-x:auto;">
 <p> Contents of Sun0 </p>
</div>

<div id="Fri0" class="tabcontent">

<!-Friday-->
<div style="overflow-x:auto;">
 <p> Contents of Sun0 </p>
</div>