There is no טהרה without טבילה and time does not purify (חייב כרת until she immerses כראוי in a מקוה ראוי), the מצוה to immerse בזמנה on the night of the 8th day so as not to nullify פריה ורביה (and the Rama's leniency of ליל שבת, the חפיפה close to the טבילה), the prohibition to immerse by day משום סרך בתה (never the 7th, nor the 8th or 9th by day) and the חידוש of the כלות before the חופה, cases of אונס (the 8th by day permitted, never the 7th), and the בדיעבד (עלתה לה טבילה, but she does not unite before night) — a practical and modest subject, to be learned from the Rav, the balanit and the moret kalla (Shulchan Aruch, Yoreh De'ah 197 — 5 se'ifim)
הַנִּדָּה וְהַזָּבָה וְהַיּוֹלֶדֶת לֹא עָלְתָה לָהֶן טְבִילָה עַד שֶׁיִּטְבְּלוּ כָּרָאוּי, וַאֲפִלּוּ לְאַחַר כַּמָּה שָׁנִים. וּכְשֶׁבַּעְלָהּ בָּעִיר מִצְוָה לִטְבֹּל בִּזְמַנָּהּ, שֶׁלֹּא לְבַטֵּל מִצְוַת עוֹנָה. וְאֵינָהּ טוֹבֶלֶת בַּיּוֹם, מִשּׁוּם סֶרֶךְ בִּתָּהּ.
The נדה, the זבה and the יולדת do not leave their טומאה until they immerse כראוי, even after several years. When the husband is in town, it is a מצוה to immerse בזמנה (the night of the 8th), so as not to nullify פריה ורביה ; but she does not immerse by day, משום סרך בתה.
Shulchan Aruch, Yoreh De'ah 197:1-3 (summary)LEVEL 01
The vocalized Hebrew text of the 5 se'ifim with a fluent English translation: no טהרה without טבילה (time does not purify, חייב כרת), the מצוה to immerse בזמנה on the night of the 8th and the leniency of ליל שבת, the prohibition to immerse by day משום סרך בתה and the חידוש of the כלות, cases of אונס and the בדיעבד — explained with practical cases, always referring to the Rav, the balanit and the moret kalla.
📖 StudyLEVEL 02
In-depth pilpul: אין טהרה אלא בטבילה (§1) — time does not purify, the כרת, the טבילה כראוי and the מקוה ראוי (link to hilchot mikvaot, simanim 198, 201) ; the טבילה בזמנה ופריה ורביה (§2) — the מצוה of the night of the 8th, the leniency of ליל שבת (ר״ח / בה״ג / סמ״ג) and its conditions, and the חפיפה סמוך לטבילה as the rationale of the restrictions ; the גזרה of סרך בתה (§3) — why not by day even on the 8th or 9th, and the חידוש of the כלות before the חופה ; the hierarchy of אונס (§4) — the 8th by day permitted, never the 7th ; and the בדיעבד — עלתה טבילה even when done improperly, but לא תשמש עד הלילה ותסתיר טבילתה (§5). חקירות and נפקא מינות.
📖 StudyLEVEL 03
Comparative tables (the principle אין טהרה אלא בטבילה, the טבילה בזמנה and ליל שבת, סרך בתה and the כלות, the אונס and the בדיעבד), golden rules, classic pitfalls (by day / by night, the 7th / the 8th, חפיפה סמוך לטבילה, לא תשמש עד הלילה) and memorization of the 5 se'ifim — always under the guidance of the Rav, the balanit and the moret kalla.
📖 StudyLEVEL 04
The shitah of the Tzemach Tzedek of Lubavitch and the Chabad mesorah on the time of the טבילה, immersion at night and the minhagei tevila — a living Chabad domain, presented at the level of principle while referring for conduct to a Chabad Rav —, then the halacha le-ma'aseh according to the Beit Yosef, the Rama, the Shach, the Taz, the Sidrei Tahara, the Chochmat Adam, the Aroch haShulchan and contemporary pesak (Taharat haBayit, Shevet haLevi, Badei haShulchan). Every conclusion refers to the Rav, the balanit and the moret kalla.
📖 StudyThe founding principle of Siman 197 is that there is no טהרה without טבילה: the passage of time does not purify. The נדה, the זבה and the יולדת never leave their טומאה — even after years — until they have immersed כראוי in a מקוה ראוי (a valid mikveh). One who comes to them before she has immersed כראוי is חייב כרת. This conveys the gravity of immersion and of its validity (the details of the mikveh are in simanim 198 and 201). The conduct le-ma'aseh — when and how to immerse — is learned from the Rav, the balanit and the moret kalla.
When the husband is in town, it is a מצוה to immerse in its time — the night of the eighth day — so as not to nullify פריה ורביה, even for a single night. The Rama permits immersing ליל שבת if she could not immerse earlier (and only when the husband is in town) ; but if she could have immersed earlier (after childbirth, or if the husband returned only ערב שבת), there is a יש אומרים that forbids it, and the custom depends on the locale. The חפיפה (the combing and preparation) must be close to the טבילה — אין מרחיקין הטבילה מן החפיפה. The question of ליל שבת or יום טוב, and that of the חפיפה on Friday, are put to the Rav.
The טבילה is done at night, never by day. It is forbidden to immerse on the 7th day (by day) ; and even if she waits until the 8th or 9th day, she does not immerse by day — משום סרך בתה : lest the daughter, seeing her mother immerse by day, imitate and immerse on the 7th day itself, not distinguishing that her mother immersed after the seven and not on the 7th. The Rama adds a חידוש : the כלות (brides) who immerse before the חופה may immerse by day (since they only join the חתן at night) ; but after the חופה, their law reverts to that of other women. The practice is transmitted by a moret kalla and the balanit.
Where there is אונס (intense cold, fear of thieves, closing of the city gates…), she may immerse on the 8th day by day (מבעוד יום) ; but on the 7th day she does not immerse by day even באונס. And בדיעבד: if she immersed on the 8th by day without אונס, עלתה לה טבילה (the טבילה counts) ; and even the 7th by day, עלתה לה טבילה. Nevertheless, per the Rama, she does not unite — even on the 8th — before night, and she conceals her טבילה from her husband until night, lest she come to immerse by day. Every case of אונס (travel, security, mikveh hours) is put to the Rav.