Before we begin, I want to explain why I present the development of the halachah the way I do, rather than simply writing the bottom line. At the end of the day, you aren't required to know the verses, the Mishnah, the Gemara or the machlokot among the Rishonim, though there is certainly a beautiful mitzvah in studying them. You can keep the mitzvah of taharat hamishpachah by reading a basic halachah book — Darchei Taharah, Torat HaTaharah and the like. The practical halachah is what is critical.

So why go into the details?

The first reason: many of us want to know why. Why did the Shulchan Aruch write this halachah? What is the source?

The main reason, though: when we study this thoroughly, we learn something valuable. We learn how to ask questions.

What does that mean? Those excellent halachah books, which let us look up quickly what to do, simply cannot cover every situation, because they have to be concise. But if I study the development of the halachah, I understand the underlying principles. And if I understand the underlying principles, I can recognize when an unusual situation calls for a question. That can help us find room for leniency in mitigating circumstances, and also recognize prohibitions we were never explicitly taught.

To the matter at hand — when a hefsek taharah is performed

This article begins the subject of the timing of the hefsek taharah.

First, it is critical to establish on which day a woman may perform a hefsek taharah. Very important: this article addresses the timing of the hefsek taharah, not the timing of the seven clean days. Although the seven clean days generally do begin on the night following the hefsek, as we will discuss, that is not always so.

The dispute in the Gemara

The Mishnah in Masechet Niddah records a machloket about which day a niddah may perform a hefsek taharah. The underlying question: from what point can we say that the hefsek genuinely proves the bleeding has stopped? The idea is that if the hefsek is performed too close to the onset of bleeding, too large a window remains between the hefsek and immersion, and bleeding may have occurred in that window without the woman's knowledge.

The dispute among the Rishonim

On the basis of the Mishnah and the Gemara, the Rishonim dispute whether Rabbi's lenient view can be relied on.

The Rambam holds that a hefsek taharah may not be performed on the day the bleeding began. The reason: there is an assumption that on the day bleeding began, ma'yanah patuach (מַעְיָנָהּ פָּתוּחַ) — the uterus is open, and it is not possible that the bleeding stopped that same day.

The Rashba holds that she may perform a hefsek even on that day.

Why is this relevant at all?

In most cases the whole dispute looks academic — an ordinary period does not last a single day. But a woman can certainly experience a single day of intermenstrual bleeding. In that case the question arises: may she perform a hefsek taharah that same day?

The ruling of the Shulchan Aruch

The Shulchan Aruch rules that a hefsek may be performed on the day the bleeding began. But in that case she must also perform a moch dachuk as the Rashba described, and without it her hefsek is invalid.

The Rema notes that although lechatchilah she must perform a moch dachuk when the hefsek is on the day the bleeding began, b'dieved, as long as she performed the hefsek close to bein hashemashot, the hefsek is valid even without an accompanying moch. "Close to bein hashemashot" is generally understood as any time after minchah ketanah.

Practical halachah

A woman may perform a hefsek taharah on the day the bleeding began. In that case she must also perform a moch dachuk. If for some reason she cannot perform a moch dachuk, or forgot, she should ask.

All the Rishonim agree that the seven clean days may not begin on the day of the hefsek taharah.