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Bathing and Mikveh:
It is a Mitzvah to bathe one’s body in hot water on Erev Pesach in honor of Yom Tov.
Mikveh: It is a customary to immerse in a Mikveh on Erev Pesach in order to purify oneself for the Regel.
When on Erev Pesach is one to go to Mikveh?
One is to only go to Mikveh after midday, or at the very least past the 5th hour of the day. If one cannot immerse at that time then he may immerse anytime in the morning. One is to cut his nails prior to immersing.
Taking a shower after Mikveh:
It is permitted to shower after Mikveh, although some are stringent not to do so. Nevertheless, on Erev Shabbos [and Erev Yom Tov] according to all it is better not to do so, in order so one not completely wash off the Mikveh water from his body, as will be explained next.
Drying oneself after Mikveh:
Based on the teachings of the Arizal one should not dry the Mikveh water off his body after immersing. Practically we are particular to dry ourselves with a towel after immersion and one who desires to follow the directive of the Arizal is to leave some part of his body not dried. It suffices for one to leave his feet wet in order to fulfill this directive.
Sparks of Kabala
The purification affected by a Mikveh:
There is a tradition from the Kabalists that there are four hundred aspects of impurity that reside upon an impure person. In order to remove these 400 aspects of impurity one must immerse in a Mikveh of 40 Seah. The waters represent Chesed and hence have the ability to purify the body. One’s entire body must be immersed in the Mikveh simultaneously as if even one hair is protruding from the water the impurity remains on that hair and then spreads to the rest of the body [upon coming out of the water].
 Admur 529:2 regarding every Erev Yom Tov
 Sefer Hasichos 5696 p. 129 and 5703 p. 74
 The following sources state this regarding Erev Shabbos: Kanfei Yona 1:95; Mishnes Chassidim Yom Hashishi 7:1; Shlah p. 138a last line in name of Kanfei Yona; Likkutei Mahrich 2 p. 7b; See also Likkutei Dibburim 3:568
 Mateh Ephraim 625:14 in name of Shaar Hakavanos and Peri Eitz Chaim; Chayeh Adam 138:5; Kaf Hachaim 581:82; Alef Lamagen 581:121; Likkutei Mahrich 2 p. 7b in name of Siddur Rav Shabsi
The reason: As from that time and on the radiance of Shabbos begins to shine. [Mateh Ephraim 625:14 in name of Shaar Hakavanos and Peri Eitz Chaim]
 Mateh Ephraim 625:14
 See Kaf Hachaim 260:1 and Mateh Ephraim 625:13; Shlah 138 that the nails are to be cut prior to Mikveh. To note however of the custom of the Rebbe Maharash which would cut the nails after Mikveh. His reasoning was because at this time the nails are softer. [Story heard from Rav Zalman Shimon Dworkin]
 Sheivet Halevy 7:33; Piskeiy Teshuvos 88:8; Even regarding a Nidda the Rama Yoreh Deah 201:75 rules that only some opinions are stringent and so is the custom. However others argue on Rama and rule doing so is permitted even by a Nidda. [See Gr”a ibid; Darkei Teshuvah 202:332; Lechem Vesimla 122]
 Piskeiy Teshuvos 260:1 in name of Sheivet Halevy that so is the custom of the world to avoid doing so despite it being allowed from the letter of the law. See also Tiferes Adam 1:28 which rules one should never shower after Mikveh.
 Based on Kaf Hachaim 260:8
 As the water is considered holy with the spirit of Shabbos [and Yom Tov] and it is thus proper for the body to absorb it. [ibid] However see Kaf Hachaim 260:8 which writes that this only applies if one did not immerse more than once, otherwise he has already washed away the main Mikveh water. Nevertheless he concludes that even these second waters of the Mikveh has some holiness and is thus not to be dried.
 The holiness of that leftover water from the Mikveh will then subsequently spread to the rest of the body. [Rebbe Shaarei Halacha Uminhag 1:131]
 Shaarei Halacha Uminhag 1:131
 Chesed Leavraham Mayaan 2:59