Add Day.prev_date/Day.next_date and use them for navigation in Day page.

[plomtask] / templates / day.html

{% extends 'base.html' %}

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