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6. Doing Hefsek Taharah on Shabbos:
[Being that washing only minority of one’s limbs with water heated before Shabbos is permitted,] therefore in a place where there is no [accepted] custom [otherwise], a woman is allowed to begin her seven clean days on Shabbos, being that [to do so] she only needs to wash her private part and between her thighs. However, she must be careful not to wash with a cloth, so she not come to squeeze it. [She may use either cold water or hot water that was heated before Shabbos for the purpose of this wash. Those areas however that are accustomed to being stringent not to do Hefsek Taharah on Shabbos are to be stringent. Practically, the custom is to perform a Hefsek Taharah even on Shabbos.]
A woman may clean her area for a Hefsek Taharah as doing so is only washing minority of one’s body.
How to use the hot water to do the Hefsek Taharah:
Place the water that was heated from before Shabbos into a clean and dry vessel, add cold water to it to cool it down, and you may then use it on the body for the Hefsek Taharah.
 Admur 326:1; Masas Binyamin 5; M”A 326:3; Michaber Y.D. 199:6 regarding Yom Tov and the same applies towards Shabbos; Shach 199:12; M”B 326:6
 After a married woman menstruates, she must separate from her husband until she has counted seven clean days, meaning days that she has not seen any blood. To begin the count, she must wash herself the afternoon prior to her wanting to begin the seven-day count and then check herself to verify that the blood has stopped flowing. Thus, this washing may be done on Shabbos in order to have the first of the seven days begin after Shabbos.
 Admur ibid; Rama ibid
 Admur ibid; Masaas Binyamin ibid, brought in Shach ibid
 Maasas Binyamin ibid, brought in Shach ibid and Machatzis Hashekel ibid; Kitzur Dinei Taharah 4:7
 Brought in Maasas Binyamin ibid; M”A ibid; Shach ibid; M”B ibid; implied from Admur ibid
The reason for this custom: As women do not know how to properly wash themselves without transgression, as they may come to use water heated on Shabbos, and they may come to squeeze the cloth. [Maasas Binyamin ibid, brought in Shach ibid] Alternatively, the reason is because they do not want their Tevila to fall on Motzei Shabbos and thus have the Chafifa pushed to Friday. [Shach ibid; Machatzis Hashekel ibid]
 Taharas Yisrael 197:14; Darkei Teshuvah 197:14; Aruch Hashulchan 199:19
 See Piskeiy Teshuvos 326 footnote 44