Imparfait on Saturday, but Passé Composé on Sunday?

Question

Ok, Team Kwiziq. Au secours! À l'aide!

"Samedi dernier, il y avait du brouillard et il faisait froid et humide."

- We're describing the weather, setting up the condition. Ergo: l'imparfait seems the perfect tense here.

"Dimanche, il a fait moins froid mais il a plu toute la journée."

- In context, we're implying the day following "samedi dernier," still describing the weather, setting up the condition, so why do we jump to the passé composé? It makes sense to me to use it the sentence "Mon frère et moi sommes sortis dans le jardin pour ramasser des escargots," as it happened on that particular Sunday, doesn't feel habitual, doesn't feel like it was on ongoing event...

Nonetheless why not "Dimanche, il faisait moins froid mais il pleuvait toute la journée." Or even better, "Dimanche, il faisait moins froid mais il a plu toute la journée." ???

Answer

I'm not sure, but I think the point is that the first sentence sets the context for the whole text. The changes in weather on Sunday are just seen as individual events, rather than creating a new context.

Perhaps it would have been possible to use the imperfect instead, it would just give a different effect. I suppose it would create more of a feeling of separation between the two days.