<style>
 .sunday{background-color:red;}
 .monday{background-color:yellow;}
 .friday{background-color:aqua;}
 .saturday{background-color:aqua;}

</style>

<script type="text/javascript" src="/sites/SiteAssets/jquery-latest.min.js"></script>
 <script type="text/javascript">
 $(document).ready(function(){

 interval = setInterval(ready, 1000);
 });

function ready()
 {
      $sun = $("table.ms-acal-month>tbody tr td:nth-child(1)");
  $mon = $("table.ms-acal-month>tbody tr td:nth-child(2)");
  $fri = $("table.ms-acal-month>tbody tr td:nth-child(6)");
  $sat = $("table.ms-acal-month>tbody tr td:nth-child(7)");

     $sun.each(function(){
       $(this).addClass('sunday');
        $(this).find("div").addClass('sunday');
      });

$mon.each(function(){
       $(this).addClass('monday');
        $(this).find("div").addClass('monday');
      });

$fri.each(function(){
       $(this).addClass('friday');
        $(this).find("div").addClass('friday');
      });

$sat.each(function(){
       $(this).addClass('saturday');
        $(this).find("div").addClass('saturday');
      });


}
 </script>