On page 305 of Genki , you can see that Mary starts off each paragraph with a day of the week and the particle that follows is different. Any reason why friday is listed w/ NI vs the other days that all have WA? Could you replace the WAs with NI?
The text goes as follows:
金曜日に日本人のともだちとこうえんにいきました。こうえんでともだちとはなしました。それから、レストランへいきました。たくさんたべました。
土曜日は一人でおてらへいきました。たくさんみせがありました。みせでおまんじゅうをかいました。
日曜日はおそくおきました。おかあさんもおそくおきました。わたしはあさテレビをみました。それから、おかあさんとひるごはんをたべました。ごごは日本語をべんきょうしました。本もよみました。
土曜日 and 日曜日 are used with は because she is changing the topic to those days respectively. That is, since she was talking about the day before in the previous paragraph, she is now changing the topic to talk about what she is doing on Saturday (or Sunday) in the current paragraph.
Using に with them wouldn't be wrong, but it sounds better with は in this context because she's listing off the days in order and then dedicates a paragraph to describing what she did. If only に were used, the days wouldn't sound as important.
She also could have used は with 金曜日, but it doesn't matter as much as the others because she wasn't talking about anything before, so there's no need to emphasize 金曜日.
