If a woman sees menstrual blood a little after iftar in Ramadan, and she does not know whether the menstruation began before iftar or after it, does she have to fast that day?

The summary of the answer is: If a woman sees menstrual blood and doubts whether it appeared before or after sunset, the default in this case is that the blood appeared after sunset, based on the principle, "The origin of every incident is estimated by its nearest time." Accordingly, her fast for that day is valid, and she must make up the Maghrib prayer after she becomes pure, because the entry of the time obligates the menstruating woman to perform it.

This is supported by what is stated in the Fiqh Encyclopedia and a fatwa by Sheikh Al-Shinqiti.