Gemara

(Rav Yehudah): People who dwell in huts or traverse wildernesses lack life. Their children are not their own.

Support (Beraisa - Eliezer Ish Biriya): It says about their daughters "Arur Shochev Im Kol Behemah."

(Ula): This is because they do not have baths.

(R. Yochanan): It is because women tell each other when it is time to immerse after being Nidah.

They argue in a case when there is a river nearby.

Nidah 66b: A woman came in front of R. Yochanan. Whenever she returned from immersing she would see blood.

R. Yochanan: Perhaps there is an Ayin Ra'ah on you. Have Bi'ah with your husband near the river [where you immerse, before other women see you coming].

67b - Question: May a woman do Chafifah (shampoo and clean herself) at night and then immerse?

Answer (Rav Ada): Once, the Reish Galusa quarreled with his wife [on the day of Leil Tevilah, i.e. the night she was destined to immerse]. Rav Nachman appeased her that night;

She thought that she cannot immerse now, for she had not done Chafifah. He told her '[you can do Chafifah now and immerse now, for] surely you have slaves [to heat the water]!'

68a: The Halachah is, she may do Chafifah during the day and immerse at night.

Contradiction: The Halachah is, she must do Chafifah at night.

Answer: One discusses when it is possible. The other discusses when it is impossible.

Rishonim

Rosh (Hilchos Mikva'os 37): Rashi explains that it (Chafifah during the day) is possible on a weekday, but not if she immerses on Motza'ei Shabbos. The She'altos (Acharei Mos 96) explains that when it (Chafifah) is possible at night, i.e. on a weekday, she should do so. When it is impossible at night, i.e. on Shabbos or Yom Tov, she does Chafifah during the day.

Hagahos Ashri (5:3): We apply to the daughters of hut dwellers "Arur Shochev Im Kol Behemah" because they sense when the others immerse. Therefore, it is the custom of women to be modest on the night of Tevilah, and not go blatantly.

Poskim

Shulchan Aruch (YD 198:48): Some say that a woman should be modest on Leil Tevilah. It is the custom of women to conceal the Leil Tevilah, and not go amidst turmoil or in front of people, lest people sense. It says about one who does not do so "Arur Shochev Im Behemah."

Teshuvas Maharshal (6): Maharam Padova and his son established that women should go to the Mikveh house at night, and shampoo and immerse at night, because the Mikveh house was in the Chatzer Beis ha'Keneses, and men would sense when women come to immerse. It is Midas Chasidus to conceal Leil Tevilah. The Roke'ach derived this from Eruvin 55b. Rashi explained that a Rasha pursues them and is isolated with them, for the place of Tevilah was far away. However, we could say that it was for modesty, lest men look at them, and there will be Hirhurim on both sides. Some women, due to great shame, concealed Leil Tevilah from their husbands, and found an excuse to delay it until Motza'ei Shabbos, when they would be forced to go at night, when people will not notice. Early Chachamim were adamant to shampoo during the day, lest she rush. Now they enacted that women delay a full hour before immersing, so they will not rush. A certain Chacham told women that this is forbidden, and the Tevilah does not permit them and the children are Mamzerim. The Chacham became angry, therefore he erred terribly.

Yehudah Ya'aleh (YD 217): A certain modest woman works in a store. Is it proper to delay her Tevilah until Shabbos, when the store is closed, lest people know when is Leil Tevilah? I answer like Dagul me'Revavah (YD 197), that if she cannot immerse until Shabbos due to stringencies of Poskim, she may immerse then. [Even though on Shabbos we immerse only if it is the first opportunity,] this is her first opportunity! Here, the stringency is based on Eruvin 55b and the Rema.

Shulchan Aruch (24): Some women do not eat meat on the day they will immerse, lest strands get stuck in her teeth and she will not know. This is a good custom.

Taz (25): The custom is to eat meat if it is Shabbos or Yom Tov, just she must clean her teeth extra well.

Question: A certain woman eats with others (e.g. parents), and if she will not eat meat, they will realize that it is Leil Tevilah. The Rema says that it is proper to conceal this. Should she eat meat?

Answer (Beis She'arim YD 280): Yehudah Ya'aleh went overboard. Rashi explains that when there is a river nearby, it allows covert immersion, for it is close to her. He connotes that when there are no Mikva'os in the city, and they go to immerse in the river in the field, we are concerned for publicity and seclusion. When there are Mikva'os in the city, there is no concern. Even Rashi is concerned only for a Rasha who will follow her. There is no such concern for her father, who may be secluded with her. Also for other relatives, we do not establish a particular person to be a Rasha. Nidah 67b and YD 197:3 connote that a woman does not conceal Leil Tevilah from her household. (There is concern lest a daughter immerse during the day, like she saw her mother do.) Also, it seems that the concern is only if they know when she goes to the Mikveh, but not at the meal beforehand. Also, refraining from meat does not show that it is Leil Tevilah. Perhaps she does not want to eat meat! Therefore, she should refrain from meat, especially since perhaps they will not notice. The Safek lest they notice does not override the custom to refrain from meat.

Shevet ha'Levi (5:118): Darchei Teshuvah (Sof Siman 198) cites Beis She'arim, who holds that concealing Leil Tevilah does not justify immersing on Shabbos. This is wrong. We permit immersing on Shabbos for a small need, and all the more so to conceal the Leil Tevilah. The Maharshal permitted Chafifah at night l'Chatchilah due to this! However, I say to delay immersing for the sake of concealing it only in cases like that of Yehudah Ya'aleh, in which closing the store would be a great publicity, but not merely if one eats at her parent's table, especially if they did not yet fulfill Peru u'Rvu. Rather, she should try to conceal it as much as possible.

Yabi'a Omer (5 YD 19): Beis She'arim says that she need not conceal it from her household. This is wrong. The Roke'ach, Maharshal and Rema forbid due to Hirhur. Even Rashi mentioned seclusion with a Rasha only to explain why the Gemara said that children are Mamzerim. The Ben Ish Chai (Rav Pe'alim 2 YD 33) permits immersing during the eighth day to conceal the Leil Tevilah even from the family.

Si'ach Yitzchak (419): If there is only one Mikveh, and if a woman will go during the day to do Chafifah she will expel the men, they will know that it is her Leil Tevilah. Maharam Padova and his son enacted that they do Chafifah at night in such a case, lest men come to Hirhur Aveirah. The Maharshal, Rema and many others approved. Nidah 67b connotes that she may do Chafifah in the house. However, perhaps the Reish Galusa had a Mikveh in the house, and she immersed immediately after Chafifah.

Seridei Esh (2:44): If a woman is embarrassed to immerse in the Mikveh, she may not immerse in the river. Firstly, perhaps men will see her. This causes her to rush, and perhaps she will not immerse properly (Nidah 66b, YD 198:34). The same applies where she fears lest Nochrim make problems. All women conceal the Leil Tevilah. Even though other women are in the Mikveh house, she is not ashamed due to them, for all came to immerse.

Mishneh Halachos (16:52): There was a Mikveh in a building with a Beis Midrash. There was only one entrance. When a woman came, all the men knew that it was Leil Tevilah. Sometimes a woman asked the men where is the Mikveh! The Rema applies to this Arur Shochev Im Behemah. Also, perhaps the first thing the women see after immersing will be male Yisre'elim, which leads to a spirit of Zenus. There are four reasons to conceal Leil Tevilah. Rashi in Eruvin explains that people hear, and a Rasha might be secluded with her. If the women go in cars to the Mikveh, this does not apply. The Roke'ach learns from Nidah 66b. A woman used to see blood after immersing due to Ayin Ra'ah. He explains that this is the concern also in Eruvin, if other women will sense when is Leil Tevilah. It is called Arur Shochev Im Behemah, for this is like animals, which are not ashamed to have Bi'ah in front of others. The Maharshal holds that if men know, there will be Hirhur. Beis She'arim distinguishes between the household and strangers. Virtuous women would conceal Leil Tevilah even from their husbands. It seems that it is due to Hirhur, lest they come to have Kishuy (erection) intentionally. Perhaps we should be concerned also if Leil Tevilah becomes known, that women will worry lest men see them, and they will rush Tevilah.

Yabi'a Omer (ibid.): A woman was too embarrassed to immerse at night because she has older children in the house. May she immerse on the eighth day? Tzemach Tzedek (66) said that he never saw women immerse on the seventh day, except for Shabbos, when they go early to conceal the Leil Tevilah from their households. He said that this must be abolished. It seems that he permits on the eighth day. Sidrei Taharah (197:14) and Chachmas Adam (118:3) permit this. Kapei Aharon says that this is the custom in Yerushalayim, where people do not live in separate houses and Chatzeros, and neighbors would realize when a woman returns from immersing at night, which is a great sin. Pischei Teshuvah (197:11) brings from Chamudei Daniel that the Heter to immerse on the eighth day is only when there is an Ones that affects the entire city to immerse at night. Nowadays, modest women find excuses to conceal the Leil Tevilah. However, the Rambam (Hilchos Isurei Bi'ah 4:8) says that one may immerse at night due to illness, even though it does not affect the entire city. Many Acharonim permit even for a personal Ones. One may rely on them.

Note: When there were Taharos, women had to know when their neighbors were Temei'im. A man is lashed for Bi'ah with his wife when she was Muchzekes Nidah among her neighbors (Kesuvos 72a)! However, there was no need to tell neighbors the day before Leil Tevilah.