It’s not meant to be a representative day. The characters even hang the lampshade well, telling the newbies that this was an insane day, and the rest will be easier. It being someone’s first day is a common writing technique, it gives the writers a chance to explain things to the audience. The idea is that these are the sort of things that may happen on any given day in the ER. They just all happen on that day for the sake of showing the audience.
The mass casualty event is especially interesting. It’s directly based on the hospital closest to the 2017 Las Vegas shooting.