Use different exceptions throwing different HTTP codes for different cases.

[plomtask] / templates / calendar.html

{% extends 'base.html' %}

{% block content %}
<form action="calendar" method="GET">
from <input name="start" value="{{start}}" />
to <input name="end" value="{{end}}" />
<input type="submit" value="OK" />
</form>
<ul>
{% for day in days %}
<li><a href="day?date={{day.date}}">{{day.date}}</a> ({{day.weekday}}) {{day.comment|e}}
{% endfor %}
</ul>
{% endblock %}