Fixed #26217 -- Added a warning about format strings to WeekArchiveView docs.

    * Uses a default ``template_name_suffix`` of ``_archive_week``.

    * The ``week_format`` attribute is a :func:`~time.strptime` format string

      used to parse the week number. The following values are supported:

      * ``'%U'``: Based on the United States week system where the week

        begins on Sunday. This is the default value.

      * ``'%V'``: Similar to ``'%U'``, except it assumes that the week

        begins on Monday. This is not the same as the ISO 8601 week number.

    **Example myapp/views.py**::

        from django.views.generic.dates import WeekArchiveView

        <p>

            {% if previous_week %}

                Previous Week: {{ previous_week|date:"F Y" }}

                Previous Week: {{ previous_week|date:"W" }} of year {{ previous_week|date:"Y" }}

            {% endif %}

            {% if previous_week and next_week %}--{% endif %}

            {% if next_week %}

                Next week: {{ next_week|date:"F Y" }}

                Next week: {{ next_week|date:"W" }} of year {{ next_week|date:"Y" }}

            {% endif %}

        </p>

    In this example, you are outputting the week number. The default

    ``week_format`` in the ``WeekArchiveView`` uses  week format ``'%U'``

    which is based on the United States week system where the week begins on a

    Sunday. The ``'%W'`` format uses the ISO week format and its week

    begins on a Monday.  The ``'%W'`` format is the same in both the

    :func:`~time.strftime` and the :tfilter:`date`.

    However, the :tfilter:`date` template filter does not have an equivalent

    output format that supports the US based week system. The :tfilter:`date`

    filter ``'%U'`` outputs the number of seconds since the Unix epoch.

    In this example, you are outputting the week number. Keep in mind that week

    numbers computed by the :tfilter:`date` template filter with the ``'W'``

    format character are not always the same as those computed by

    :func:`~time.strftime` and :func:`~time.strptime` with the ``'%W'`` format

    string. For year 2015, for example, week numbers output by :tfilter:`date`

    are higher by one compared to those output by :func:`~time.strftime`. There

    isn't an equivalent for the ``'%U'`` :func:`~time.strftime` format string

    in :tfilter:`date`. Therefore, you should avoid using :tfilter:`date` to

    generate URLs for ``WeekArchiveView``.

``DayArchiveView``

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