Is ghusl obligatory when menstrual bleeding and brown discharge stop for a full day before the usual period ends, or must one wait for the entire period to elapse?

A woman's regular period can increase or decrease. If she sees purity during the time of her regular period, she must hasten to perform ghusl, and she will have the rulings of pure women. It is not permissible for her to wait until the days of her regular period have passed after seeing purity, due to the saying of Ibn Abbas, may Allah be pleased with them both: "It is not lawful for her to delay ghusl if she sees purity for even an hour." If the blood returns, she reverts to being in a state of menstruation.