[Dilettante_Pro]

<div class="month" data-month="3">
   <div class="week">
      <div class="day">
         3
      </div>
   </div>
</div>
<div class="month" data-month="4">
   <div class="week">
      <div class="day">
         13
      </div>
   </div>
</div>
<script>
var days = document.querySelectorAll('.day');
for(var i=0; i < days.length; i++) {
    alert( days[i].parentNode.parentNode.getAttribute('data-month'));
}
</script>