1,092. Hefsek Taharah on Shabbos, Yom Kippur, Tisha b'Av, etc.

159:4 A woman should always act stringently by checking for her hefsek taharah with a cloth that is inserted vaginally and left in for the entire twilight period. An examination such as this removes any trace of doubt.

159:5 Some places have the practice that if the time for putting on white undergarments falls on Shabbos or yom tov, then it is delayed until afterwards. This is because not every woman knows the details of the prohibitions against bathing and wringing out clothes on Shabbos. In places where they are accustomed to act leniently, if she bathes in cold water she may even wash her whole body. In hot water, however, she must be careful only to wash her genitals and between her thighs, using only water that was heated before Shabbos or yom tov. She must also be careful about the prohibition against wringing out, and therefore not wash using a cloth but only her hand. On Yom Kippur she may not wash at all; she may only wipe herself very well. This is an extenuating circumstance when washing is not possible, such as were discussed in 159:2. After this, she changes into the white undergarments. On Tisha b'Av and when she is sitting shiva, she likewise may not wash, only wipe, plus she may not change into the fresh underwear, only previously-worn undergarments that she has checked to ensure that they have no blood spots. However after shiva, even though one may not bathe the entire thirty-day period of mourning, nevertheless she may wash a little for this and she may put on the white undergarments.