Symbolism, Imagery, Allegory
Nancy's diary symbolizes the future that she'll never have. The purpose of a diary is to write down the events of today to read in the distant future, which is a heartbreaking idea in Nancy's case, since the reader knows what's going to happen to Nancy and her family.
On the night of her death, Nancy dutifully notes down the boring events of her day, unintentionally making the vital point that, on the last night of your life, when you have no future, no event is boring.