At what hour of the day may a hefsek taharah be done?

The ideal time — late afternoon

As mentioned, where a hefsek is performed on the day the bleeding began, it is done in the late afternoon hours, from minchah ketanah until sunset.

That is in fact the ideal time to perform a hefsek taharah regardless of how many days have passed since the onset of bleeding, as the Shulchan Aruch writes.

When it is difficult — a morning "backup" hefsek

Some women find it very hard to perform a hefsek taharah at that hour, whether because they are not at home or because they are busy around that time and may forget. In such cases it is recommended that the woman perform a backup hefsek in the morning. Then, if she forgot or could not perform a hefsek at the ideal time, her morning hefsek is valid and she can begin counting the seven clean days the next day.

This is in line with the Rema, who writes that b'dieved a morning hefsek is valid, as long as it was not performed on the day the bleeding began.

It must be before sunset

Although lechatchilah the hefsek is performed in the afternoon, it is critical to understand that a hefsek taharah must be performed before sunset.

Some women want to fulfill this mitzvah lemehadrin, as close to sunset as possible. That is not necessarily advisable. If a woman performs a hefsek and sees that it is not clean, we want her to have enough time to try again if she wishes. And if she forgets, we do not want her to miss sunset altogether.

What if the hefsek was performed after sunset?

B'dieved, if a woman performed a hefsek taharah at the moment of sunset or a few minutes after, despite having tried to do it on time, she should ask.

Some rabbanim will say it is valid in certain circumstances, on the strength of combined grounds for leniency, such as the fact that the precise moment of sunset is itself disputed. Others, Chabad among them, hold that it is not valid under any circumstances.

Why does it matter so much that the hefsek precede sunset?

Let us turn to Masechet Niddah to understand.

Masechet Niddah discusses the Kutim, a group whose Jewish lineage was in doubt and who were unaware of many of the nuances of Jewish law. They are considered bo'alei niddot: although the women attempted to purify themselves from the tum'ah of niddah, their lack of halachic precision prevented them from achieving taharah.

One of their mistaken practices was beginning the seven clean days on the day of the hefsek. If a Kuti woman performed a hefsek taharah on Sunday afternoon, she would count Sunday as one of her seven clean days. Monday would already be day 2, and so on.

All the Rishonim agree that the seven clean days may not begin on the day of the hefsek. The seven clean days are separate from the day of the hefsek, and so the Shulchan Aruch rules.

What that means in practice

So if a woman performs a hefsek after sunset, which is the halachic end of the day, then when she begins counting seven clean days she is in effect including the day of the hefsek as a clean day. And that is not permitted.

Practical halachah

A hefsek taharah (when it is not on the day the bleeding began) should lechatchilah be performed in the late afternoon hours, between minchah ketanah and sunset. If that is difficult for any reason, a woman may perform a backup hefsek in the morning (valid only if the hefsek is not on the day the bleeding began).

If she could not or forgot to perform a hefsek at the ideal time, she can rely on her morning hefsek.

The hefsek must be performed before sunset. If a woman mistakenly performed one at the moment of sunset or a few minutes after, she should ask her rav (or a yoetzet, who will bring the question to a rav, since this is a matter of ruling) whether it is valid.