Gene recounts the events of that entire day down to the smallest and most mundane details. Why might the author have chosen to do this?

Gene's ability to recall all of the events of the day of Finny's death is very significant; it is like that whole day is frozen in time, and his discussing it in such detail is almost like he is reliving it. This is the only time in the whole book when Gene talks about a meal he had at school, the conversation over the meal in the dining hall, and what he did, hour by hour, on one school day. He talks about his class schedule, where he was when and what he did during that time, and evokes, for the first time in the book, what an ordinary day was like at Devon.

That he recalls everything in such detail before he finds out about Finny's death is strange, because it is almost like he knows about it from the night before, when Gene started to recall things in incredible detail. Otherwise, he would have had to go back in his memory and tack down everything that had happened that day, if he could remember, after he found out about Finny.