Replace first character of string in loop

On the page, the calendar columns are displayed with abbreviations, such as "sat" instead of "saturday".

<div class="dayClass">sat, 14 may</div>
<div class="dayClass">sun, 15 may</div>
<div class="dayClass">mon, 16 may</div>

to be like below

<div class="dayClass">Saturday, 14 may</div>
<div class="dayClass">Sunday, 15 may</div>
<div class="dayClass">Monday, 16 may</div>

I wrote this code by searching
But I can’t just change the word "day" and the date will also be replaced

window.onload = function findAndReplaceDayName(){
  var elements = document.querySelectorAll('.dayClass');
for ( var i=elements.length; i--; ) {
  var text = elements[i].innerText;
   if(text.slice(0,3) == 'sat' ){
    elements[i].textContent = "Saturday"
  }else if(text.slice(0,3) == 'sun'){
elements[i].textContent = "Sunday";
    }else if(text.slice(0,3) == 'mon'){
elements[i].textContent = "Monday";
}}

But the result is this

<div class="dayClass">Saturday</div>
<div class="dayClass">Sunday</div>
<div class="dayClass">Monday</div>

Can you help me to change only the day and not the date.
Thankful
Footnote: I am new to both programming and English language

>Solution :

window.onload = function findAndReplaceDayName() {
  var elements = document.querySelectorAll('.dayClass');
  for (var i = 0; i < elements.length; i++) {
    var text = elements[i].innerText;
    if (text.indexOf(',') !== -1) {
      var dayName = text.slice(0, text.indexOf(',')).trim();
      var date = text.slice(text.indexOf(',') + 1).trim();
      switch (dayName) {
        case 'sat':
          dayName = 'Saturday';
          break;
        case 'sun':
          dayName = 'Sunday';
          break;
        case 'mon':
          dayName = 'Monday';
          break;
        // add more cases for other days if needed
      }
      elements[i].textContent = dayName + ', ' + date;
    }
  }
};