JSFiddle - React, Tailwind, and code Playground

HTML

<p>Click to change day</p>
<button type="button" id="prev" name="btnPrev">prev</button>
<button type="button" id="next" name="btnNext">next</button>
<p id="todayField"></p>

JavaScript

var date = new Date();
var index = date.getDay();
var date = new Date();
    var days = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"];
    document.getElementById("todayField").innerHTML = days[date.getDay()];

    function next() {
      var days = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"];
      var nextIndex = index + 1 > 6 ? 0 : index+1;
      index = nextIndex
        document.getElementById("todayField").innerHTML = days[nextIndex];
    }

    function prev() {
      var days = ["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"];
      var nextIndex = index - 1 < 0 ? 6 : index-1;
      index = nextIndex
      console.log(nextIndex)
      document.getElementById("todayField").innerHTML = days[nextIndex];
    }
 document.getElementById("prev").addEventListener("click", function() {
   prev();
 })
 document.getElementById("next").addEventListener("click", function() {
   next();
 })