JSFiddle - React, Tailwind, and code Playground

HTML

<select id="select" onchange="myFunction()">
  <option value="Пн">Пн</option>
  <option value="Вт" selected>Вт</option>
  <option value="Ср">Ср</option>
  <option value="Чт">Чт</option>
  <option value="Пт">Пт</option>
  <option value="Сб">Сб</option>
  <option value="Вс">Вс</option>
</select>
<p>Рабочий</p>

JavaScript

var button = document.querySelector('button');
var p = document.querySelector('p');
var select = document.querySelector('select');

function myFunction() {
	if (select.value == "Сб" || select.value == "Вс")
  	p.innerHTML = "Выходной";
   else
   	p.innerHTML = "Рабочий";
};