see http://anmonteiro.com/2015/08/jekyll-related-posts-revamped/

jekyll_related_posts.html

{% assign RELATED_POSTS_THRESHOLD = 3 %}

<ul>
  {% assign related_post_count = 0 %}
  {% for post in site.related_posts %}
    {% if related_post_count == RELATED_POSTS_THRESHOLD %}
      {% break %}
    {% endif %}
    {% for tag in post.tags %}
      {% if page.tags contains tag %}
        <li>
          <h3>
            <a href="{{ site.baseurl }}{{ post.url }}">
              {{ post.title }}
              <small>{{ post.date | date_to_string }}</small>
            </a>
          </h3>
        </li>
        {% assign related_post_count = related_post_count | plus: 1 %}
        {% break %}
      {% endif %}
    {% endfor %}
  {% endfor %}

  {% assign posts_left = RELATED_POSTS_THRESHOLD | minus: related_post_count %}
  {% unless posts_left == 0 %}
    {% for post in site.related_posts %}
      {% if posts_left == 0 %}
        {% break %}
      {% endif %}

      {% assign already_related = false %}
      {% for tag in post.tags %}
        {% if page.tags contains tag %}
          {% assign already_related = true %}
          {% break %}
        {% endif %}
      {% endfor %}
      {% unless already_related %}
        {% assign posts_left = posts_left | minus: 1 %}
        <li>
          <h3>
            <a href="{{ site.baseurl }}{{ post.url }}">
              {{ post.title }}
              <small>{{ post.date | date_to_string }}</small>
            </a>
          </h3>
        </li>
      {% endunless %}
    {% endfor %}
  {% endunless %}
</ul>