JSFiddle - React, Tailwind, and code Playground

HTML

<span class="fc-day-top"><a href="#">12</a></span>
<span class="fc-day-top"><a href="#">14</a></span>

CSS

.bookedDate {
  width: 30px;
  height: 30px;
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: red;
  border-radius: 50px;
  pointer-events: none;
}

JavaScript

const bookedDates = document.querySelectorAll('.fc-day-top');

bookedDates.forEach(function (bookedDate) {
	bookedDate.classList.add('bookedDate');
});