The official medical term for the phenomenon that you're describing is "menstrual synchrony." When girls or women live together in close proximity, such as in a dormitory, they often find that their periods all start at around the same time each month. It is likely that this is caused by pheromones, or chemicals, that are constantly being released from our bodies. Though you cannot see or smell these chemicals, they can actually be detected by others' bodies and cause subtle hormonal shifts. When women live together and therefore spend a great deal of time with one another, they are constantly exposed to each other's pheromones. Over time, this can influence the timing of their menstrual cycles.