Gemara

(Rav): When a Nidah immerses in the proper time (after the seven days), she must immerse at night. If she immerses afterwards (day eight or later), she may immerse by day or at night.

(R. Yochanan): In any case, she must immerse at night, due to Serach Bitah (concern lest her daughter think that her mother used to immerse during the seventh day, and she will do similarly).

Rav retracted to hold like R. Yochanan;

(R. Chiya bar Ashi citing Rav): A Nidah must immerse at night, in the proper (first possible) time or afterwards, due to Serach Bitah.

In Narash (a place), Rav Idi enacted that women immerse on the eighth day, due to (danger at night due to) lions;

Rav Acha bar Yakov, Rav Yehudah and Rava enacted this in their respective cities due to thieves, the cold and Nochri gatekeepers.

Question (Rav Papa): Nowadays, women are stringent to consider themselves Safek Zavos (like R. Zeira taught). They should be allowed to immerse on the seventh (clean) day!

Answer (Abaye and Rava): They may not due to R. Shimon's teaching:

(Beraisa): "V'Achar Tithar" -- after all the (seven clean) days (that a Zavah must count). Tum'ah may not interrupt in the middle;

R. Shimon expounds "v'Achar Tithar" -- after an action, she is Tehorah. (She may immerse in morning of day seven). However, Chachamim forbid doing so, lest she come to a Safek.

Rishonim

Rambam (Hilchos Isurei Bi'ah 11:4): Benos Yisrael in every place are stringent and after any sighting of blood, even a drop like a mustard seed, they count seven clean days after the blood stops, like a Zavah Gedolah. She immerses on the eighth night, even though she is a Safek Zavah, or on the eighth day if there is difficulty (to immerse during the day).

Rosh (Hilchos Mikva'os 36): R. Shimon says that Chachamim forbade immersing on day seven, lest she have Bi'ah, which is a Safek. Therefore, nowadays women may not immerse on day seven. One may not immerse during the day (even) on day eight or nine, lest her daughter think that she immerses on day seven, and she will do similarly, and come to a Safek.

Suggestion: This applied only then, when they immersed from Nidah mid'Oraisa and there was concern lest her daughter immerse on day seven of Nidah, which is invalid, but nowadays (that women immerse after seven clean days) it is merely mid'Rabanan (to immerse on day seven), so we are not concerned.

Rebuttal (Rosh): This is wrong. Rava enacted to immerse on day eight when it is problematic at night, and R. Zeira's stringency had already spread, like we find from Rav Papa's question (66a). The Rashbam was stringent to forbid Tevilah before dark. R. Tam says that it suffices to delay Tevilah until close to dark, so it will be dark before she returns home, and there is no concern.

Poskim

Shulchan Aruch (YD 197:3): She may not immerse on day seven.

Beis Yosef (YD 183 DH u'Mah she'Chosav b'Zavah): Here, the Tur explains like the Rambam. In Siman 197, he explains like Rashi! It seems that here he teaches that mid'Oraisa, she may immerse on day seven.

Beis Yosef (DH u'Mah she'Chasav Aval): Nowadays, all women (who saw blood) are Safek Zavos. They could immerse on the seventh day, but Chachamim decreed not to, lest they come to a Safek.

Beis Yosef (DH v'Chasav): The Rashba says that women may not immerse on day seven, for if she establishes herself to be Tehorah, perhaps she will have relations, and perhaps she will see later in the day. Even though this is a decree due to a decree, Chachamim were stringent because there is Kares. Further, they decreed even against immersing during the day on day eight, unless there is danger or other reasons not to immerse at night. Some are lenient to permit immersing on day seven. R. Shimon forbids only Bi'ah during the day. This is wrong. Rav Yehudah and Rava enacted to immerse on day eight (when it is problematic at night), and they were after R. Zeira's stringency.

Note: After R. Zeira, we always require seven clean days, like Zavos. Therefore, we cannot say that they enacted on day eight due to Nidos, who may not immerse on day seven. However, Rava said (66a) that not everyone accepted R. Zeira's stringency, and Rav Yehudah was even before Rava!

Beis Yosef (ibid.): The Ra'avad wrote in the name of the She'altos that if she immersed on day seven, it was invalid. This is unreasonable. Chachamim were not so stringent, for there are many Sefekos. It is a problem only if she was a Zavah, and if she will have Bi'ah right after immersing (i.e. during the day, which Yisre'elim generally do not do - PF) and if she will see blood during the day. We do not disqualify Tevilah b'Di'eved due to such unlikely concerns. Only the Ba'al ha'Ma'or permits to immerse l'Chatchilah on day seven, and the Rashba refuted him.

Shach (6): The Shulchan Aruch forbids even close to dark even if she will return home after dark, like the Rashbam. The Bach forbids even heading to the Mikveh during the day, i.e. if she bathes and shampoos in the house and goes to immerse. If the bathhouse and Mikveh are in one place, and she heads there an hour or two before dark and returns home after dark, even though the bathhouse and Mikveh are near her house, we are not concerned about her daughter, for her daughter know that she bathes and shampoos there and does not immerse until dark. The Agur writes that the custom in Ashkenaz is close to dark. Perhaps this is like R. Tam. It seems that this is the custom. In any case, one should be stringent, i.e. on day eight, due to Serach Bitah. On day seven one may not immerse at all shortly before dark, and one must protest against those who do.

Shulchan Aruch (ibid.): Even if she waits until day eight of nine, she may not immerse during the day due to Serach Bitah.

Shach (8): The same applies to a woman without a daughter. Chachamim did not distinguish. Maharam Lublin (71) was unsure about one who immersed improperly, and must immerse again two or three days later, if she may immerse during the day, and if she must count seven clean days due to Serach Bitah, lest she think that her mother saw blood and immersed after two or three days. He did not answer. I am stringent l'Chatchilah about the first question, and lenient b'Di'eved. Surely, she need not wait seven clean days. The only decree due to Serach Bitah is lest she immerse during the day. We find that one who erred about the days may immerse after four days (196:12). One who immersed improperly may immerse at any time.

Meshiv Davar (2:44): A woman did not immerse from Nidah for a long time, and did not separate from her husband. She requested to immerse on the eighth day, for she is ashamed to immerse at night amidst other women. If we do not permit her, she will continue to have relations without immersing. In some places Chachamim permitted in order to help people repent. We accept Korbanos from sinners, to encourage them to repent (Chulin 5a), even though it says "Zevach Resha'im To'evah." We accept a Chatas for an Aveirah that he does not transgress b'Mezid, but not if he is a Mumar to that Aveirah, even if at the time he transgressed b'Shogeg. The Chazakah is that he will not repent from it. Here, since she and her husband transgressed until now, we do not believe that she will observe laws of Nidah from now and onwards. When her shame ceases, she will find excuses why she cannot immerse. Even if she repents, her husband will not want to wait until she becomes Tahor, and rather than argue with him, she will return to the Isur she was accustomed to. Therefore, we should not compromise with her unless she and her husband accept Teshuvah for the past and acceptance for the future.

Meshiv Davar (ibid.): If the woman initially came and said that her Yetzer Ra overpowered her not to immerse at night due to shame, and if she does not immerse she will probably transgress Nidah, could we permit a small Isur (to immerse during the day) lest she come to a big Isur? Rishonim argued about this. If one sees that his Yetzer Ra is overpowering him, he should go to a place where he is not known and do what he desires (Chagigah 9a). Rashi and Tosfos explain that he should sin in private, and not come to Chilul Hash-m. R. Chananel, and Rashi in Mo'ed Katan citing Rav Hai Gaon, explain that surely he may not sin. Going away will help him to avoid sinning! They do not permit a small Aveirah to prevent a big Aveirah. Beis Shmuel is like Rashi and Tosfos. However, not all light Aveiros are the same. Perhaps we do not permit a small Isur to avoid a great Isur if there is concern lest others stumble. We may not permit something to help this woman that may cause many to stumble. Sometimes Chachamim were lenient about immersing during the day in a place where all women did so (e.g. due to danger). Here, all women immerse at night. If they see her immerse during the (eighth) day, they may think that one may immerse during the seventh day. They don't know that she is different, and is ashamed to immerse at night. Also, since it depends on her will, other women will say the same. One must consider well before permitting an Isur due to an Isur. Sometimes a doctor decides to cut off an aching hand lest it endanger the body, and sometimes he decides that it is better to suffer the pain and not lose the hand.

Rema: Kalos who immerse before the Chupah may immerse during the day, for they do not come to the Chasan until the night. After the Chupah, she is like regular women.

Shach (9): In a place where Kalos immerse on day seven, or even for us, if she erred during the count, and immerses right after seven days, she may not immerse during the day. This is not due to Serach Bitah, rather, lest she see and be Soser her count, and retroactively she was a Zavah.

R. Akiva Eiger: Where Kalos immerse on day seven, she may not immerse before sunrise. B'Di'eved after dawn it helped. The Tur says that mid'Oraisa she must wait until sunrise. This is difficult. The Torah permits her from dawn!