There is a lot here, and it is worth reading all of it. The practical halachah on this one is hard to grasp without the background.

This article clarifies when the seven clean days may begin relative to the onset of bleeding, and in particular why an Ashkenazi woman waits five days and a Sephardi woman four.

You are surely familiar with the need to wait four days (for Sephardi women) or five days (for Ashkenazi women) before performing a hefsek taharah. Now we will learn why it is necessary.

The source — Matan Torah

When Am Yisrael received instructions to prepare for Matan Torah, they were commanded to refrain from marital relations for three days. Why? For the sake of the women.

A man who has an emission holds the status of a baal keri, and remains tamei for one day only. So to receive the Torah in taharah, the men needed to refrain from relations for one day.

But a woman who has relations acquires the status of poletet shichvat zera. Halachah assumes her body may expel viable seed. As long as the seed is viable, if her body expels it, with or without her knowledge, tum'ah results.

How long does the seed remain viable?

The Gemara records a machloket about how long the seed remains viable. The Sages hold it is a 72-hour window, and so the Shulchan Aruch rules. That is, halachically, once 72 hours have passed since relations, a woman no longer holds the status of poletet shichvat zera.

So that the women could receive the Torah in a state of taharah, marital relations were forbidden for three days before Matan Torah.

Why does this tum'ah exist?

Tum'ah and taharah are divine concepts that the human mind cannot fully grasp. Still, it seems to me that the concept of poletet shichvat zera becomes easier to hold onto if we see that every kind of tum'ah arises where there is a loss of potential.

With death, the gravest form of tum'ah, the potential lost is plainly life itself. With niddah, the potential of pregnancy. With a baal keri (above), potential is lost too, even where the emission resulted in a pregnancy. And with poletet shichvat zera: if the body expels seed, there is a kind of loss of potential, even if, say, the woman was using contraception and could not have conceived at all.

What difference does it make today?

Today, with Matan Torah long past and the third Beit HaMikdash still awaited, one could ask: what difference does it make whether a woman is a poletet shichvat zera? Why should this status of tum'ah matter?

The answer: this form of tum'ah interferes with a woman's ability to count seven clean days.

The connection to the seven clean days

Let us look at the sources:

"And when she is cleansed of her discharge, she shall count for herself seven days, and after that she shall be pure." (ויקרא טו, כח)

The Gemara in Masechet Niddah (37a) teaches that Rabbi Shimon derived a halachah from the word "and after" — "and after that she shall be pure." He learned from it that a woman who experienced uterine bleeding may immerse only if her seven clean days were consecutive, with no interruption of tum'ah.

An example: a woman experienced uterine bleeding, performed a hefsek taharah, and counted three of her seven clean days, only to wake and find she is bleeding again. She cannot resume the count once the bleeding stops. She must begin counting afresh.

Now, we learned from Rabbi Shimon that tum'ah from blood voids a woman's seven clean days. The Gemara wants to establish whether other forms of tum'ah void them too, such as the tum'ah of poletet shichvat zera. The question comes up several times in Masechet Niddah, and in most instances we find the view that poletet shichvat zera does interfere with counting the seven clean days.

Most Rishonim agree that this is the halachah, and so the Shulchan Aruch rules.

The practical consequence

This means a woman cannot begin counting seven clean days until she no longer holds the status of poletet shichvat zera. That ensures her seven clean days will not be interrupted by an expulsion of seed. The concern is that a woman might expel seed without knowing it, unwittingly interrupt her seven clean days, and immerse on the wrong day.

Exactly how long to wait? The Shulchan Aruch against the Rema

On exactly how long a woman must wait between marital relations and counting seven clean days, there is a striking dispute between the Shulchan Aruch and the Rema, which is reflected in Sephardi and Ashkenazi practice today.

The ruling of the Shulchan Aruch

A woman who had marital relations and afterwards experienced uterine bleeding cannot begin counting seven clean days until the fifth day from the relations. That means she can perform a hefsek taharah on the fourth day.

An example: a woman who had relations on Motzaei Shabbat may perform a hefsek taharah on Wednesday before sunset, and her seven clean days begin on Thursday.

A woman who did not have marital relations and experienced uterine bleeding, however, need only wait for the bleeding to end. So if she bled for only two days, she can perform a hefsek taharah immediately.

A further leniency in the Shulchan Aruch

The Shulchan Aruch goes one step further with an additional leniency: even where a woman did have marital relations and then experienced bleeding, she may expel the seed manually, or sit in a warm bath to expel it, and perform a hefsek taharah immediately.

The ruling of the Rema — two stringencies

The Rema brings two stringencies in the name of the Terumat HaDeshen.

First stringency — five days

The woman must wait until the sixth day from relations to begin counting seven clean days. That is, she may perform a hefsek taharah on the fifth day.

The Rema explains the reason: if a woman had relations during bein hashemashot (the interval between sunset and nightfall), she may mistakenly think the relations occurred technically during the day when in fact they occurred at night, which is the next halachic day. If so, she would begin counting the seven clean days a day too early.

Note: several yoatzot halacha have observed that the Rema's case is especially relevant in Ashkenazi lands, where bein hashemashot lasts about an hour, against roughly 20 minutes in Eretz Yisrael.

Second stringency — five days even without relations

Even where there were no marital relations, the woman must still wait until the fifth day from the onset of bleeding to perform a hefsek taharah. The Rema stresses that although this is a stringency, it is the established custom and a woman should not depart from it.

Practical halachah

Sephardi women

May perform a hefsek taharah from the fourth day from the onset of bleeding.

Chacham Ovadia Yosef rules according to the plain sense of the Shulchan Aruch: a woman may count the four days from the last time she had marital relations. Although most women do not get a clean hefsek before the fourth day from the onset of bleeding in any case, there are in practice many women with short cycles, and if they are Sephardi they may count the four days from the relations.

An example: a Sephardi woman who had relations on Motzaei Shabbat and bled on Monday and Tuesday may perform a hefsek taharah on Wednesday before sunset.

Had that woman wanted to try a hefsek on Tuesday (assuming all the bleeding had stopped), according to some poskim she could have rinsed internally and then performed a hefsek on Tuesday. Since relying on internal rinsing to expel the husband's seed is a machloket among the poskim, a woman should consult a posek before using that method.

Ashkenazi women

May perform a hefsek taharah only from the fifth day from the onset of bleeding.

This holds whether or not she had relations immediately before the bleeding. As long as she and her husband were permitted to each other before the bleeding began, she must wait 5 days.

If, however, she and her husband were already forbidden to each other — if she had a gynecological procedure that rendered her a niddah, say — she counts the 5 days from the time the procedure rendered her a niddah.

This is critical information for a woman who began counting seven clean days and then saw a stain that voided them. The Shach explains that she does not need to wait another 5 days before performing a new hefsek taharah. She may perform a new hefsek taharah as soon as possible, because the 5 days are counted from the time she became forbidden to her husband.

Internal rinsing (to expel seed and allow an earlier hefsek taharah) is not relevant for Ashkenazi women.

When may an Ashkenazi woman be permitted to follow Sephardi practice?

It's worth knowing that Ashkenazi women have some familiarity with the Sephardi ruling, because in extreme cases, with a ruling from a rav, an Ashkenazi woman may be permitted to follow it. For example:

An Ashkenazi woman who feels her circumstances are mitigating, and who could physically perform a hefsek taharah before the fifth day from the onset of bleeding, should certainly seek halachic guidance.