[j0hnik]

<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>
[].forEach.call(document.querySelectorAll('.day'), function(el){ 
	while (el = el.parentNode) {
		if (el.dataset.month) {
			console.log(el.dataset.month);
			break;
		}
	}
});
</script>

более универсальное решение