My textbook says that passe compose is used when it's an event or action that happens over a period of time (a day, a week, etc) and imparfait for shorter periods (i.e hours). Samedi, Marion fete son anniversaire. Elle fait une boum. Tous ses copains viennent. It would turn to "Samedi dernier, Marion a fete son anniversaire. Elle a fait une boum." But for the third and fourth sentence, they have nothing to do with time. How would I change it?