Jekyll/Liquid Templating: How to group blog posts by year?

If you want to break it down by year, here's the code:

{% for post in site.posts  %}
    {% capture this_year %}{{ post.date | date: "%Y" }}{% endcapture %}
    {% capture next_year %}{{ post.previous.date | date: "%Y" }}{% endcapture %}

    {% if forloop.first %}
    <h2 id="{{ this_year }}-ref">{{this_year}}</h2>
    <ul>
    {% endif %}

    <li><a href="{{ post.url }}">{{ post.title }}</a></li>

    {% if forloop.last %}
    </ul>
    {% else %}
        {% if this_year != next_year %}
        </ul>
        <h2 id="{{ next_year }}-ref">{{next_year}}</h2>
        <ul>
        {% endif %}
    {% endif %}
{% endfor %}

If you want to break it down to year and months it can be achieved like this:

{% for post in site.posts  %}
    {% capture this_year %}{{ post.date | date: "%Y" }}{% endcapture %}
    {% capture this_month %}{{ post.date | date: "%B" }}{% endcapture %}
    {% capture next_year %}{{ post.previous.date | date: "%Y" }}{% endcapture %}
    {% capture next_month %}{{ post.previous.date | date: "%B" }}{% endcapture %}

    {% if forloop.first %}
    <h2 id="{{ this_year }}-ref">{{this_year}}</h2>
    <h3 id="{{ this_year }}-{{ this_month }}-ref">{{ this_month }}</h3>
    <ul>
    {% endif %}

    <li><a href="{{ post.url }}">{{ post.title }}</a></li>

    {% if forloop.last %}
    </ul>
    {% else %}
        {% if this_year != next_year %}
        </ul>
        <h2 id="{{ next_year }}-ref">{{next_year}}</h2>
        <h3 id="{{ next_year }}-{{ next_month }}-ref">{{ next_month }}</h3>
        <ul>
        {% else %}    
            {% if this_month != next_month %}
            </ul>
            <h3 id="{{ this_year }}-{{ next_month }}-ref">{{ next_month }}</h3>
            <ul>
            {% endif %}
        {% endif %}
    {% endif %}
{% endfor %}

It is only a matter of where do you make the cut on the loop.

These previous solutions are fantastic but luckily in late 2016, Jekyll added a group_by_exp filter that can do this much more cleanly.

{% assign postsByYear =
    site.posts | group_by_exp:"post", "post.date | date: '%Y'" %}
{% for year in postsByYear %}
  <h1>{{ year.name }}</h1>
    <ul>
      {% for post in year.items %}
        <li><a href="{{ post.url }}">{{ post.title }}-{{ post.date }}</a></li>
      {% endfor %}
    </ul>
{% endfor %}

Documentation can be found on the Jekyll Templates page.

It can be done with much, much less Liquid code than in the existing answers:

{% for post in site.posts %}
  {% assign currentdate = post.date | date: "%Y" %}
  {% if currentdate != date %}
    <li id="y{{currentdate}}">{{ currentdate }}</li>
    {% assign date = currentdate %} 
  {% endif %}
    <li><a href="{{ post.url }}">{{ post.title }}</a></li>
{% endfor %}

This will return exactly the HTML specified in your question:

<li id="y2013">2013</li>
<li><a href="/2013/01/01/foo/">foo</a></li>
<li id="y2012">2012</li>
<li><a href="/2012/02/01/bar/">bar</a></li>
<li><a href="/2012/01/01/baz/">baz</a></li>

---

However, this is not the optimal solution, because the year numbers are "only" list items as well.
It's not much more Liquid code to put the year into a headline and to begin a new <ul> for each year's posts:

{% for post in site.posts %}
  {% assign currentdate = post.date | date: "%Y" %}
  {% if currentdate != date %}
    {% unless forloop.first %}</ul>{% endunless %}
    <h1 id="y{{post.date | date: "%Y"}}">{{ currentdate }}</h1>
    <ul>
    {% assign date = currentdate %}
  {% endif %}
    <li><a href="{{ post.url }}">{{ post.title }}</a></li>
  {% if forloop.last %}</ul>{% endif %}
{% endfor %}

The generated HTML:

<h1 id="y2013">2013</h1>
<ul>
<li><a href="/2013/01/01/foo/">foo</a></li>
</ul>
<h1 id="y2012">2012</h1>
<ul>
<li><a href="/2012/02/01/bar/">bar</a></li>
<li><a href="/2012/01/01/baz/">baz</a></li>
</ul>

---

You can also group by month and year instead (so that the headlines are February 2012, January 2012 and so on).

To do this, you just need to replace date: "%Y" (in the second line of both above examples) by date: "%B %Y".
(%B is the full month name, see the documentation)

Some solutions above are very complex but then as @Trevor pointed out that we can levarage Jekyll's group_by_exp filter. Also I liked the solution but what I needed was grouped by Year and then inside that list grouped by Month. So, I tweaked it a little bit.

{% assign postsByYear = site.posts | group_by_exp:"post", "post.date | date: '%Y'" %}
    {% for year in postsByYear %}
      <h1>{{ year.name }}</h1>
      {% assign postsByMonth = year.items | group_by_exp:"post", "post.date | date: '%B'" %}

      {% for month in postsByMonth %}
        <h2>{{ month.name }}</h2>
        <ul>
          {% for post in month.items %}
            <li><a href="{{ post.url }}">{{ post.title }}-{{ post.date }}</a></li>
          {% endfor %}
        </ul>

      {% endfor %}
    {% endfor %}