如果您删除了 $('.target').show(); 无论单击什么，都将显示所有
注意：我在初始视图中添加了一个显示：无

$(function() {
  $('#indexed').click();
});
$('.single').click(function() {
  $('.target').hide();
  $('#div' + $(this).attr('target')).show();
});

.target { display:none }

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div class="container days">
  <div class="menu">
    <a id="indexed" class="single" target="1">today</a>
    <a class="single" target="2">Yesterday</a>
    <a class="single" target="3">Two Days Ago</a>
  </div>

  <section class="target_box">
    <div id="div1" class="target">content 1</div>
    <div id="div2" class="target">content 2</div>
    <div id="div3" class="target">content 3</div>
  </section>
</div>

<section class="target_box">
    <div id="div1" class="target">content 1</div>
    <div id="div2" class="target" style="display:none;">content 2</div>
    <div id="div3" class="target" style="display:none;">content 3</div>
  </section>

看起来你必须先隐藏一些内容。因此，只有内容1可见。