UPDATE:
I updated this file: /templates/ja_landscape/html/mod_articles_category/isotope.php
to make the category Un jour, une rando be active by default
If you want other category be active, you can edit above file and change the id: 12 to id of category you want:
<button class="filter-button<?php echo $cat->id == 12 ? ' active' : ''; ?>" data-filter=".isotope-<?php echo $cat->id ?>">
and:
filter: ".isotope-12",