我当时也遇到了类似的问题，这个人的解决方案帮了我：How to Change the Day of Week Format。
但是，我需要替换整个文本，e。g.不只是在它后面添加一个字母，所以我这样做了：

/* set the visibility of the Th:cell to hidden */
.ant-picker-body table thead tr > th:nth-child(1) {
   visibility: hidden;
}

/* add the new content, turn visibility ON, reset margins and replace
   text using css's pseudo selector*/
/* first item */
.ant-picker-body table thead tr > th:nth-child(1)::after {
   content: 'DOM';
   position: absolute;
   visibility: visible;
   left: 2px;
   top: 0;
}
.ant-picker-body table thead tr > th:nth-child(2) {
   visibility: hidden;
}
   
/* do the same for all other cells from the names of the days of the
   week to the seventh day*/
.ant-picker-body table thead tr > th:nth-child(2)::after {
  content: 'SEG';
  position: absolute;
  visibility: visible;
  left: 4px;
  top: 0;
}

在这里，它工作得很好，希望这有帮助