Include Jekyll posts by tag following the desired tag order

home.html

---
layout: default
---

<div class="home">
  {% include posts-by-tag.html tag_name="tag2" %}
  {% include posts-by-tag.html tag_name="tag1" %}
</div>

posts_by_tag.html

{% for tag in site.tags %}
{% capture tag_name %}{{tag | first}}{% endcapture %}
{% if tag_name == include.tag_name %}

<h2 class="post-list-heading" id="{{ tag[0] | slugify }}">{{ tag_name }}</h2>
<ul class="post-list">
  {% for post in tag[1] %}
  <br>
  <h3>
    <a class="post-link" href="{{ post.url | relative_url }}">
      {{ post.title | escape }}
    </a>
  </h3>
  <div class="post-meta">{{ post.date | date: "%b %-d, %Y" }}</div>
  </li>
  {% endfor %}
</ul>

{% endif %}
{% endfor %}