For POST /day, do not default to a valid date if none provided.

[plomtask] / templates / day.html

{% extends 'base.html' %}

{% block content %}
<h3>{{day.date}} / {{day.weekday}}</h3>
<form action="day?date={{day.date}}" method="POST">
comment: <input name="comment" value="{{day.comment|e}}" />
<input type="submit" value="OK" />
</form>
{% endblock %}