The death of a parent should be one of the climaxes of a person’s life. We could, prior to reading the whole book believe that the story is going to narrate Mersault’s life before the death of his mother, how this event changed his life and the future without his loved one. Those inferences would be wrong. But then why did Camus start the novel that way, if it was not a marking point for the main character? Being this an existentialist novel, things really do not matter, they simply happen. Most existentialist emphasize that the present is the only truth. There is really no need for the readers to be set in a setting; you will read this novel because it is your free will and the present gives you the opportunity. So Camus therefore exposes you to Mersalut’s life in the present, what has just happened. Some people might take death as a huge event; others just see it as one more day.
“And just then it crossed my mind that one might fire, or not fire—and it would come to absolutely the same thing” (PG. 37)
Mersault’s life is flashing before his eyes, right before a confrontation with some Arabs. When a non existentialist would quote something like “when your life flashes in front of your eyes, make sure it is worth watching”, Camus replaces it with “when life flashes in front of your eyes, it is just another day”. That is finally why Camus starts the novel where he chooses to, because simply it is just another day.