Gemara

(Rav Sheshes): "La'Ayil" includes the ram of the Kohen Gadol on Yom Kipur.

Question: We learn this from "Oh b'Mo'adeichem"!

Rosh Hashanah 16b (R. Yitzchak): One must become Tahor during the festival - "uv'Nivlasam Lo Tiga'u".

Support (Beraisa) Suggestion: Perhaps "uv'Nivlasam Lo Tiga'u" forbids Yisre'elim to touch a Neveilah!

Rejection: Even for severe Tum'ah (of a Mes), "Bnei Aharon" are commanded, but not Bnei Yisrael are not. All the more so, Bnei Yisrael are not commanded about a light Tum'ah (Neveilah)!

Rather, "uv'Nivlasam Lo Tiga'u" forbids Yisre'elim to touch a Neveilah during the festival.

Yevamos 29b (Beraisa): If an Arusah dies, her husband does not become Tamei for her (for needs of her burial). Similarly, if he dies, she does not become Tamei for him.

Rishonim

Rambam (Hilchos Evel 2:6): "Lah Yitama" is an Aseh for a male Kohen to be Mitamei for his relatives. A Kohenes has no Isur to be Mitamei, therefore she need not be Mitamei for relatives. It is optional.

Rebuttal #1 (Ra'avad): An Arusah need not become Temei'ah for her husband. This implies that a Nesu'ah must become Temei'ah for him.

Rebuttal #2 (Ramach): If a Kohen is forbidden to become Tamei, yet he is commanded to be Mitamei for relatives, all the more so a Bas Kohen, who may become Tamei, is commanded to be Mitamei for relatives!

Ramban (Toras ha'Adam 36, in Kisvei ha'Ramban 2 p.133): The Rambam must say that the Beraisa is not precise regarding an Arusah.

Rebuttal (Sha'agas Aryeh 66): How can he say that the Beraisa teaches something totally meaningless?! If women have no Isur or Mitzvah to become Tamei, how is a Nesu'ah different?! Rather, the Rambam explains like Rashi in Yevamos, that it discusses during the festival, when Yisre'elim are commanded (even) about Tum'as Neveilah. The Ra'avad holds that the Beraisa cannot discuss during the festival, He learned from Rosh Hashanah. What forced the Beraisa to establish the verse to discuss during the festival? The verse does not hint to this at all! Rather, during the festival one must be Tahor in order to fulfill the obligatory Korbanos. One may bring Olas Re'iyah and Chagigah until the last day. One can become Tahor from Tum'as Neveilah in one day. Why is it forbidden the entire festival? Rather, the Isur is due to Shalmei Simchah, which one must eat every day of the festival. (Indeed, if one ate Shelamim today, he may touch a Neveilah and immerse, and at night he is Tahor again. However, every day he is forbidden to touch a Neveilah before eating Shelamim.) If a woman is obligated in Simchah, the Isur Tum'ah applies to her. The Rambam (Chagigah 1:1) obligates women in Simchah, like R. Zeira, so he can explain that an Arusah may not become Tamei for her Arus during the festival. The Ra'avad rules like Abaye, who exempts women from Simchah, so they are not commanded about Tum'ah during the festival. This forced him to explain that she is not commanded to be Mitamei for her Arus. If so, why does the Mishnah (Kidushin 29a) exempt women from only three LAvim (and Tum'ah during the festival is not among them)? Abaye exempts women from Bal Te'acher. He must say that this was not listed among the three because it depends on an Aseh (to bring Korbanos) from which she is exempt. Likewise, she is exempt from Tum'as Regel because she is exempt from Simchah. However, perhaps Abaye exempts women from Bal Te'acher only regarding Korbanos, but they are commanded regarding Tzedakah or Matanos Aniyim.

Rashi (Yevamos 29b v'Lo Mitam'ah): I heard that (even though a woman must be Metamei for other relatives,) an Arusah need not be Metamei for her Arus. I say that an Arusah may not become Tamei for her Arus during the festival, when (even) Yisre'elim are commanded about Tum'ah - "uv'Nivlasam Lo Siga'u"

Turei Even (Rosh Hashanah 16a DH Chayav): Abaye obligates a man to feed Shalmei Simchah to his wife. If she is Temei'ah, he is exempt. Surely she is not commanded to enable him to be Mesame'ach her!

Rambam (Hilchos Tum'as Ochlim 16:10): All of Yisrael are commanded to be Tahor on every festival, for they are prepared to enter the Mikdash and eat Kodshim. "Uv'Nivlasam Lo Tiga'u" refers only to Regalim. One who becomes Tamei is not lashed. At others times of the year, one is not commanded.

Rosh (Rosh Hashanah 4:14, in the middle of DH Gemara): R. Yitzchak bar Yehudah enacted to say 'v'Hasi'enu' (bestow on us Birkas Mo'adecha) on Rosh Hashanah and Yom Kipur. The Rosh Yeshiva in Yerushalayim told R. Meshulam to say it, based on the Yerushalmi. However, the Ge'onim R. Yitzchak ha'Levi and R. Shaltiel disagree, for it applies only on the three festivals. Birkas Mo'adecha refers to the Chagigah - "Ish k'Matnas Yado k'Virkas Hash-m Elokecha."

Rosh (Yoma 8:24): The custom is to immerse on Erev Yom Kipur. Rav Amram Gaon says that one immerses at seven hours, and then prays Minchah. Rav Sadya Gaon says that one blesses after leaving the water. I disagree. There is no hint in the Gemara or Nevi'im to this Tevilah, and it is not a custom from the Nevi'im. It is no better than taking Aravos (on Hoshanah Rabah), which we do not bless on, for it is only a custom from Nevi'im. The Tevilah is not due to R. Yitzchak's teaching, for nowadays we do not have Taharah. The entire year, a Ba'al Keri (one who had a seminal emission) need not immerse. It is not obligatory on Erev Yom Kipur, and one may not bless on it. It is a mere custom to become Tahor from Keri before Yom Kipur. A Medrash (in which angels say that Yisrael are like angels on Yom Kipur) supports the custom.

Sha'agas Aryeh (67): The Rosh connotes that if one could become totally Tahor nowadays, it would be obligatory even on Yom Kipur. If so, the same applies to Rosh Hashanah. This is wrong. even if you will say that Rosh Hashanah and Yom Kipur are included in 'Mo'adim' (the Rosh brought two opinions about this). The primary opinion holds that they are included. Menachos 91b proves that Yom Kipur is included in Mo'ed. The same applies to Rosh Hashanah. Even so, they are not included in the Chiyuv to become Tahor during the festival. We establish "uv'Nivlasam Lo Tiga'u" to discuss the festival, i.e. in order that one can fulfill Mitzvos of the Regel (Chagigah, Re'iyah and Shalmei Simchah) that cannot be done b'Tum'ah. These do not apply on Rosh Hashanah and Yom Kipur. Even when the Mikdash stands, one need not become Tahor on Rosh Hashanah and Yom Kipur. Nowadays, even if we had ashes of the Parah Adumah, like they had the days of the Amora'im, even for the three festivals there would be no need to become Tahor, since there is no Mikdash. The Rambam says that the Lav is only because we must be prepared to enter the Mikdash and eat Kodshim. He exempts from lashes because the Lav is only in order to enable bringing obligations of the Regel, which are an Aseh. We should not more stringent than for a Tahor who neglected to bring them.

Poskim

Shulchan Aruch (OC 606:4): One may immerse (on Erev Yom Kipur) whenever he wants, just it must be before night. He does not bless on the Tevilah.

Taz (4): It is good to immerse before Minchah, for then he confesses.

Magen Avraham (8): It is a foolish custom to confess (Mahariyo). Some say that the Tevilah is for Teshuvah, for even youths and virgins immerse, and one who immersed Erev Rosh Hashanah and did not see Keri since then. If so, he should immerse three times, and before Minchah. Some say to immerse after the meal before the fast, so it will be close to Yom Kipur (Maharil). I say that even one who holds that it is due to Keri obligates one who immersed Erev Rosh Hashanah and did not see Keri since then. Perhaps he had a clear or cloudy emission (not regular semen).

Mishnah Berurah (17): The custom is that even boys and girls, above Bar Mitzvah, immerse.

Kaf ha'Chayim (50): The Zohar says that the custom is to immerse, in order to pray in Taharah on Yom Kipur, and to receive the influence of Yom Kipur.

Bi'ur Halachah (88 DH v'Chen): One should remove Chatzitzos, even though one need not do so for Tevilas Ezra.

Kaf ha'Chayim (58): One should cut his nails, clean his teeth, and bathe in hot water, to clean his body. He should comb his hear. Meticulous people who do not comb their beards (lest it remove hairs) should gently separate the hairs by hand.

Rema: One needs to immerse only once, without confession, due to Keri. Pouring nine Kabim (about 13-22 liters) of water over himself also helps.

Taz (5): Nowadays the custom is to confess in the Mikveh before immersing, and to immerse three times.

Sha'ar ha'Tziyon (22): Mishbetzos Zahav says that one may not say Hash-m's name with his head uncovered, or if the water is clear and nothing separates his heart from his Ervah (and all the more so if he sees another's Ervah). Rather, he says just 'Ashmati' without Hash-m's name. I say that people who confess will not be careful and will say the usual text, so it is better to be passive and not confess. One says the full Viduy at Minchah.

Kaf ha'Chayim (53): I advise to say 'Master of the world! Chatasi, Avisi and Pashati Lefanecha. May it be Your will to pardon all of my sins in Your great mercy and Chesed, and be pleased with me.