Here is an amazing fact about human body: On average, menstrual cycle is 28 days, from the first day of menstruation to the first day of next cycle. But the menstrual period of those women who are close friends or live closely together, for example in a dorm, gradually synchronizes (occurs at the same time).
While there is some debate over “menstrual synchrony”, but Pheromones (Airborne chemical signals) and sense of smell are believe to be responsible for this interesting phenomenon.