/jp/ - Otaku Culture

i am once again asking for help with genki.
in a diary entry, the book's protagonist is writing about her weekend, and while reading i realized i'm not entirely sure about when i should use は or に when it comes to time expressions.
she starts the entry with:
>金曜日に日本人のともだちとこうえんにいきました。
notice the に after "friday".
when she starts talking about saturday, she writes:
>土曜日は一人でおてらへいきました。
she explicitly makes "saturday" the topic of her sentence, but when she talks about friday it's implicit.
to my understanding, using either に or は after "friday" ultimately means the same thing - on friday, she went with her jap friends to the park. but because there's a writing exercise where i'm supposed to use this diary entry as a model, i want to make sure i'm being accurate with my particle usage.
so, if i want to say i did something on a particular day, just like in the examples i gave, would it matter which particle i use?