1,091. Difficulty Making a Hefsek Taharah

159:2 In a case of need, such as if the woman is traveling and doesn't have even enough water to wash her genital area, it doesn't matter. In such a case, she should clean herself thoroughly with whatever is available. If she doesn't have fresh undergarments, she can put on previously-worn underwear so long as it has been checked and found to be free of blood stains.

159:3 Some authorities say that if the community has already davened maariv, even though it is still day, she can no longer make her hefsek taharah that day because for them it's already night. Other authorities permit it even if the community has already started Shabbos. One should optimally try to be careful about this but in a pinch one need not act stringently and she may make a hefsek taharah so long as it is not yet twilight. However if she personally said maariv, and certainly if she has lit Shabbos or yom tov candles, she may no longer make a hefsek taharah even if it is still day.

In the summer months, when many congregations daven maariv while it is still day, she should preferably make her hefsek taharah before this and check herself again when it gets close to twilight. This is because the main point of the hefsek taharah is the time closest to twilight. After the fact, if she only checked herself in the morning and found herself to be clean, this is sufficient.