JSFiddle - React, Tailwind, and code Playground

HTML

<div id="calendar" class="fc">
    <table class="fc-header" style="width: 100%">
        <tbody>
            <tr>
                <td class="fc-header-left"><span class="fc-button fc-button-prev fc-state-default fc-corner-left"><span class="fc-button-inner"><span class="fc-button-content">&nbsp;◄&nbsp;</span><span class="fc-button-effect"><span></span></span></span></span><span class="fc-button fc-button-next fc-state-default fc-corner-right"><span class="fc-button-inner"><span class="fc-button-content">&nbsp;►&nbsp;</span><span class="fc-button-effect"><span></span></span></span></span><span class="fc-header-space"></span><span class="fc-button fc-button-today fc-state-default fc-corner-left fc-corner-right fc-state-disabled"><span class="fc-button-inner"><span class="fc-button-content">today</span><span class="fc-button-effect"><span></span></span></span></span></td>
                <td class="fc-header-center"><span class="fc-header-title">
                    <h2>Feb 25 — Jun 7 2013</h2>
                </span></td>
                <td class="fc-header-right"></td>
            </tr>
        </tbody>
    </table>
    <div class="fc-content" style="position: relative; min-height: 1px;">
        <div class="fc-view fc-view-resourceNextWeeks fc-grid" style="position: relative;" unselectable="on">
            <table class="fc-border-separate" style="width: 100%" cellspacing="0">
                <thead>
                    <tr class="fc-first fc-last">
                        <th class="fc-resourceName">&nbsp;</th>
                        <th class="fc-id0 fc-widget-header fc-first" style="width: 13px;">Mon 2/25<br>
                            Week 9</th>
                        <th class="fc-id1 fc-widget-header" style="width: 13px;">Tue 2/26</th>
                        <th class="fc-id2 fc-widget-header" style="width: 13px;">Wed 2/27</th>
                        <th class="fc-id3 fc-widget-header" style="width: 13px;">Thu 2/28</th>
                        <th...

CSS

.monday
{
background: green;
}

JavaScript

$(function()
  {
      $('#calendar .fc-view-resourceNextWeeks thead th')
          .filter(function()
                  {
                      if($(this).text().indexOf('Mon ') == 0) return true;
                      
                      return false;
                  })
          .addClass('monday');
  });