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So take the common case today. A typical woman bleeds for five days. Common practice is to assume that we don't know if she is in her Niddah days or her Zavah days. If she was in her Niddah days she would immerse on the eighth night after the bleeding started, the third night after it stopped. But if she's a Zavah, she would immerse on the seventh day after it stopped, the twelfth day after it started. But what if she is just really a Niddah? Then her immersion "has been delayed many days" and she must immerse at night, and the first night available is the eighth night after the end of bleeding, the thirteenth night after the bleeding started.

It seems to me the key issue is the immersion not taking place at it's proper time, or at a doubtful time, and so we are stringent to avoid confusion. Since we make the assumption of blanket uncertainty in current practice, so we don't immerse during the day unless there is a good reason to do so.

It seems to me the key issue is the immersion not taking place at it's proper time, or at a doubtful time, and so we are stringent to avoid confusion. Since we make the assumption of blanket uncertainty in current practice, so we don't immerse during the day unless there is a good reason to do so.

So take the common case today. A typical woman bleeds for five days. Common practice is to assume that we don't know if she is in her Niddah days or her Zavah days. If she was in her Niddah days she would immerse on the eighth night after the bleeding started, the third night after it stopped. But if she's a Zavah, she would immerse on the seventh day after it stopped, the twelfth day after it started. But what if she is just really a Niddah? Then her immersion "has been delayed many days" and she must immerse at night, and the first night available is the eighth night after the end of bleeding, the thirteenth night after the bleeding started.

It seems to me the key issue is the immersion not taking place at it's proper time, or at a doubtful time, and so we are stringent to avoid confusion. Since we make the assumption of blanket uncertainty in current practice, so we don't immerse during the day unless there is a good reason to do so.

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Rambam seems to provide an answer הלכות אסורי ביאה ד,ו:

כָּל חַיָּבֵי טְבִילוֹת, טְבִילָתָן בַּיּוֹם: חוּץ מִנִּדָּה וְיוֹלֶדֶת--שֶׁהֲרֵי הוּא אוֹמֵר בַּנִּדָּה "שִׁבְעַת יָמִים תִּהְיֶה בְנִדָּתָהּ" (ויקרא טו,יט), הַשִּׁבְעָה כֻּלָּן בְּנִדָּה וְטוֹבֶלֶת בְּלֵיל שְׁמִינִי; וְכֵן יוֹלֶדֶת זָכָר בְּלֵיל שְׁמִינִי, וְיוֹלֶדֶת נְקֵבָה בְּלֵיל חֲמִשָּׁה עָשָׂר, שֶׁהַיּוֹלֶדֶת כַּנִּדָּה, כְּמוֹ שֶׁבֵּאַרְנוּ.

All those who are required to immerse, their immersion is in the day: other than the Niddah and one who gives birth -- as it says for the Niddah "seven days shall she be in her Niddah state", the seven days are all in the Niddah state and she immerses on the eighth night.
Also one who gives birth to a male on the eighth night, and one who gives birth to a female on the fifteenth night, as one who gives birth is like a Niddah, as we explained.

Later on in הלכות אסורי ביאה ו,כה he says:

כָּל אִשָּׁה שְׁהִיא סְפֵק נִדָּה סְפֵק זָבָה--צְרִיכָה לֵישֵׁב שִׁבְעַת יְמֵי נְקִיִּים מִסָּפֵק, וְטוֹבֶלֶת בְּלֵיל שְׁמִינִי, וְאַחַר כָּךְ תִּהְיֶה מֻתֶּרֶת לְבַעְלָהּ; וּמְבִיאָה קָרְבַּן זָבָה, וְאֵינוּ נֶאֱכָל כְּמוֹ שֶׁיִּתְבָּאַר בִּמְקוֹמוֹ.

Every woman whose state is unknown such that maybe she is a Niddah or maybe she is a Zavah -- she must sit seven clean days due to the doubt, and immerses on the eighth night, and afterwards is permitted to her husband, and brings a Zavah sacrifice, which is not eaten as we explain elsewhere.

Basically, the reason women usually immerse during the night is because the common practice today is to treat all women as if we don't know whether they are a Niddah or a Zavah. For example, if a woman bleeds for three nights, maybe all three where Zavah days, and so she must count seven clean days after the bleeding stops. She immerses on the morning of the seventh day and is permitted to her husband on the eighth night. But what if the third day was a Niddah day? Then in that specific case she must wait six more days and immerse on the seventh night after the bleeding stopped and is permitted to her husband - the night before when she would immerse if she was a Zavah. So there is allot of potential confusion as to when the right time to immerse is and the we get to this in הלכות אסורי ביאה ד,ז :

נתאחרה ימים רבים ולא טבלה--כשתטבול, לא תטבול אלא בלילה: שאם תטבול ביום, יטעו ותבוא נידה אחרת לטבול בשביעי. [ח] הייתה חולה, או שהיה מקום הטבילה רחוק ואין הנשים יכולות להגיע לו ולחזור בלילה מפני הליסטים, או מפני הצינה, או מפני שנועלין שערי המדינה בלילה--הרי זו טובלת ביום השמיני, או בימים של אחריו ביום.

If her immersion has been delayed many days -- when she immerses, she immerses only at night: that if she would immerse during the day, they will make a mistake and some other Niddah will come and immerse on the seventh day. If she was sick, or if the place of immersion was far away and the women couldn't go and return at night because of bandits, or due to modesty, or because the gates of the city are locked at night -- then they immerse on the eighth day, or on one of the following days.

It seems to me the key issue is the immersion not taking place at it's proper time, or at a doubtful time, and so we are stringent to avoid confusion. Since we make the assumption of blanket uncertainty in current practice, so we don't immerse during the day unless there is a good reason to do so.