Pourquoi est-ce qu'on écrit "...LE dimanche, on visitera l'aquarium....". Les copines ne vont pas visiter l'aquarium chaque dimanche.

Coleen, 

I don’t think this situation is covered in the lesson on days and articles, but the link below should help - see Cécile’s response to a similar question previously. 

In brief, the article is used here because you are talking about a specific day - le dimanche (de notre weekend planifié à La Rochelle) - not the coming Sunday this next weekend.

 https://kwiziq.learnfrenchwithalexa.com/questions/view/in-both-cases-when-mentioning-the-day-of-the-week-they-used-an-article-but-the-lesson-states-that-wh