When does the day begin for the calculation of the menstrual period: Is it from Maghrib prayer or from Fajr prayer?

"The term 'day' (yawm) in Islamic law has different applications. For a menstruating woman (al-haa'id), the counting of the day begins from the moment she first sees blood. So, if twenty-four hours pass from the time she sees blood, then a day and a night have passed for her."