Niddah 54 - Twilight
We have mentioned multiple times the two different counts, each with its own set of laws, that a woman has: the seven-day niddah count, and the eleven-day zivah count . We have also seen an example of a confusion that may arise when we don't know, in which of the two periods an event has occurred.
Another example of such confusion leading to a messed up count is the case of a woman who experienced a discharge during the twilight of the eleventh day of her zivah period. Twilight is that period when the day ends and the night begins, and a twilight moment might belong to the previous day, or already to the next day, since night is the beginning of the day . If her discharge occurred during the day, then it would belong to the last, eleventh day of her zivah cycle, when she only needs to wait one day after a discharge. However, if her discharge occurred at night, it would belong to the next, seven-day cycle of possible niddah. In that case, she will have to wait seven days before going to the mikveh. Since it is impossible to determine the nature of twilight, she will have not choice but to wait out seven days.
However, even after that, there is still a possible confusion. If she discharges blood on the eighth day, this might be the first day of the zivah, and then she has only to wait one day, or it could be a beginning of the new niddah period, if the previous wait was really unnecessary. Because of all these factors, she has to wait for seventeen days. However, there is never a need to wait for longer, as we mentioned in the Tractate Arachin .
Art: William-Adolphe Bouguereau - Waiting