Mikveh Night on the ninth of Av:[1]

A woman whose Mikvah night falls on Tisha B’av, must postpone it until the next night.[2] In such a case, she should perform the Chafifa [in hot water^[3]] on the day before Tisha B’av and on the night following Tisha B’av she should again bathe and clean herself, and then immerse.^[4] When Tishe Beav falls on Sunday, or on Shabbos and is differed to Sunday, then she is to perform the Chafifa on Erev Shabbos, and again two nights later on Sunday night, Motzei Tishe  Beav.[5]

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[1] Michaber 554:8; Admur 613:21

[2] Michaber ibid

[3] Derech Hachaim ibid; M”B 551:92; Kaf Hachaim 551:194

[4] M”A 554:10; M”B 554:18; Degul Merivava 551:3; Biur Halacha 551:16 “Im” based on Shach 199:9 that from the letter of the law the Chafifa must be done on Erev Tisha B’av; Kaf  Hachaim 551:198; Kitzur Dinei Taharah 5:14; Taharah Kehalacha 18:22 and so I received from Harav Asher Lemel Hakohen

Other opinions: Some Poskim rule she is to perform the Chafifa only on Motzei Tisha B’av in a case that she is able to do so. [Rama 551:16; Agudah; Beis Hillel Y.D. 199; Elya Raba 554:9; Chayeh Adam 133:19; Derech Hachaim 7; Kitzur SHU”A 122:12; Shiurei Sheivet Haleivi 199:4; Piskeiy Teshuvos 551:47; See Yoreh Deah Rama 199:4 that the Chafifa of Erev Shabbos when immersing on Motzei Shabbos is only a custom. See Kaf Hachaim 551:195]

[5] Shach 199:6; Sidrei Taharah 199:18; Taharah Kehalacha 18:20