JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.2/jquery.min.js"></script>
<div id="kalender">

  <div id="kal_line"></div>

  <ul id="days">
    <li><div class="pos1"></div></li>
    <li><div class="pos2"></div></li>
    <li><div class="pos3"></div></li>
    <li><div class="pos4"></div></li>
    <li><div class="pos5"></div></li>
  </ul>

</div>

JavaScript

Array.prototype.backwards = function(steps) {
	for (var i = 0; i < steps; i++) {
    var firstElement = this[0];
    this.shift();
    this.push(firstElement);
  }
};

$(document).ready(
  function() {
    var date = new Date();
    var days = ['sat', 'sun', 'mon', 'tue', 'wed', 'thu', 'fri'];
    
    days.backwards(date.getDay());

		$('#days').children('li').children('div').each(function(index) {
    	$(this).text(days[index]);
    });

  }
);