gistfile1.mustache

{% set has_changed = make_has_changed() %}
{% for item in foo %}
  {% if has_changed(item.date.year, item.date.month) %}
    <h1>{{ item.date.year }} {{ item.date.month }}</h1>
  {% endif %}
  ...
{% endfor %}

gistfile2.py

class HasChangedHelper(object):

    def __init__(self):
        self.state = None

    def __call__(self, *new_state):
        if self.state != new_state:
            self.state = new_state
            return True
        return False


jinja_env.globals['make_has_changed'] = HasChangedHelper