1.

a. The Rambam (Hilchos Isurei Bi'ah 6:9) writes that there is no difference between a Zavah's seeing blood during the day or at night. He writes that whether she saw at the beginning of the night or at the end of the day, the entire day is considered Tamei and it is as if the bleeding did not stop from the time of seeing until sunset at the end of the day. Therefore, if there was a Re'iyah during the day on Sunday and then another Re'iyah on Sunday night, it is considered as two consecutive days of Re'iyah and the Shimur must be on Tuesday.

b. We see from the Rambam (Hilchos Shegegos 14:2) that a mistake that a Beis Din might make would be to say that the law of Zavah applies only if she sees blood in the daytime. Therefore, the Rambam writes that if they made such a mistake, they must bring a Korban. The reason they might make such a mistake is that the Torah states, "All the days of her Zov" (Vayikra 15:26).

2. The Zavah does need a Hefsek Taharah at the end of Sunday daytime. The source for this is the Mishnah in Nidah (68a) which states that a Nidah requires Hafrashah. The Gemara (69a) teaches that a Zavah also requires "Hifrishah b'Taharah." Rashi (69a, DH v'Hacha) writes that once we know there is a Ma'ayan Pasu'ach (bleeding), we cannot give her a Chezkas Taharah until she checks herself and finds that the bleeding has stopped. The Maharsha (end of 71b) writes that even a Zavah Ketanah (who saw only one or two days) requires a Hefsek Taharah on the day that she has a Re'iyah.

3. I looked further into the question of whether a Zavah Ketanah requires Hefsek Taharah, and it is not so simple at all. I have not yet had sufficient time to look into this matter as much as I would like to, but nevertheless I will note down for the moment what I have seen on this subject.

a. Tosfos (69a, DH v'Hacha) writes: "If one would say that 'Hafrashah' is not required for a Zavah on the third day, this would make it easier to understand." The contemporary Sefer Eizehu Mekoman (which cites a lot of sources on Maseches Nidah) writes that Tosfos appears to be saying that a Zavah does not require Hefsek Taharah.

b. The Eizehu Mekoman writes that this also appears to be the opinion of the Rashba cited by the Beis Yosef in (near the beginning of YD 196, page 79a in the old editions of the Tur, DH v'Chen Mishpatah). (The Rashba is in the Toras ha'Bayis ha'Aruch, Bayis 7, Sha'ar 5, DH Ho'il, and is also cited by the Chavos Da'as 196:1 and Sidrei Taharah 196:34 DH v'Hu ha'Din.) The Rashba writes: "Therefore, even though there is a doubt nowadays concerning all of our women whether they are Nidos or Zovos, and it is possible that she is a Nidah and her Ma'ayan is open, nevertheless since she was 'Hifrishah Taharah' on the first day she has a Chazakah of being Tahor." It appears from this "Hifrishah Taharah" (i.e. Hefsek Taharah) is necessary only if she is a Nidah but not if she is a Zavah.

c. The aforementioned Sidrei Taharah writes that we see from the Rashba that when the Gemara in Nidah (68b, immediatley before the Mishnah) states, "Ho'il v'Huchzak Ma'ayan Pasu'ach," it refers only to Nidah but not to Zavah. The Sidrei Taharah writes that although a Zavah saw blood, she still does not have a Chazakah of Ma'ayan Pasu'ach. This is because the Mishnah (end of 38b) states that for all of the 11 days the Zavah possesses a Chezkas Taharah. The Sidrei Taharah writes she has a Chazakah of being "Mesulekes Domim" (she does not have bleeding; see Sidrei Taharah 190:70 who elaborates on this subject). He writes that although the Zavah now had a Re'iyah, we say that what she saw does not prove that she will see again; she will not see any more.

d. According to this, it may be possible to understand that simply because the Zavah had one Re'iyah this does not mean that she requires a Hefsek Taharah, as this is considered a random Re'iyah which is not expected to re-occur.

e. However, the view of the above sources is a big Chidush. I found that the Chidushei ha'Ritva (Nidah 30a, DH Kegon) writes explicitly that the counting of Zivah requires Hefsek Taharah. Therefore, this matter certainly requires further study.

4. After doing further research, I found more sources that discuss whether the Zavah who saw once during Sunday would need a Hefsek Taharah at the end of Sunday daytime.

a. First, here are some sources relating to the question of whether the Din of Hefsek Taharah is mid'Oraisa or mid'Rabanan. The Tosfos ha'Rosh (Nidah 69a) writes that the law of Hefsek Taharah is a "Chumra b'Alma mid'Rabanan." However, afterwards he writes that it may in fact be mid'Oraisa. See also the Or Zaru'a (beginning of Hilchos Nidah), who cites the Rashbam that Hefsek Taharah is only mid'Rabanan. (See Badei ha'Shulchan, by Rav Fivel Cohen shlit'a, near the beginning of Yoreh Deah 196, who cites these and other sources on this question).

b. One may suggest the following explanation for why Hefsek Taharah may only be mid'Rabanan. The Gemara (Nidah 57b) states that a woman is only a Nidah mid'Oraisa if she feels the bleeding in her flesh. This is recorded by the Shulchan Aruch (YD 183:1), who writes that in order to be a Nidah mid'Oraisa she must feel the blood leaving. Therefore, if a woman became a Nidah but did not feel any bleeding, one may argue that mid'Oraisa she does not require any checking.

c. The above sources relate to a Nidah and apply to why the Hefsek may not be mid'Oraisa. Everyone agrees, though, that at least mid'Rabanan she needs a Hefsek Taharah. In the case of a Zavah (even a Zavah Gedolah, who saw 3 consecutive days), we mentioned above that Tosfos (beginning of 69a) appears to be in doubt about whether she requires a Hefsek at all, and the Rashba also implies that she does not require a Hefsek. We now may suggest a reason for why the law in the case of a Zavah may be more lenient than the law of a Nidah. This is based on the Ramban on the Torah at the end of Parshas Metzora (Vayikra 15:11). The Ramban writes that, naturally, women do not experience bleeding for more than 7 days, unless there is a special flow as a result of illness. Since this is "not at the time of her Nidah" (as the Torah states in Vayikra 15:25) it must be a result of ill health similar to the phenomenon of Zov for a man. Accordingly, it may be that since the appearance of Zivah is anyway something unexpected, if she feels that it has stopped it is not necessary for her to prove this by checking.

d. However, the Shulchan Aruch (YD 196:1) writes: "The 7 days that the Zavah counts begin the day after she stops.... If she stops seeing, she checks on the day that she stops so that she can do a Hefsek Taharah...." Evidently, the Shulchan Aruch rules that a Zavah Gedolah does require a Hefsek Taharah.

e. Finally, we need to try to find out what the Halachah is in the case of a Zavah Ketanah who saw only for 1 or 2 days. I would like to suggest that according to the Rambam she does not require a Hefsek. The Rambam (Hilchos Isurei Bi'ah 6:7) starts discussing the Halachos for a Zavoa Ketanah. The Rambam mentions nothing about checking until he reaches Halachah 20, when he mentions a check for a Nidah. Then, in Halachah 21, he discusses checking for a Zavah Gedolah. The Rambam never seems to mention any checks for a Zavah Ketanah.

f. I would suggest that the reason for this is that we saw above (in c.) that Zivah is an unusual, unhealthy appearance. If it happens 3 days in a row, we say that although in general it is unusual, in this instance it has become a fixed occurance and thus requires checking in order to prove that it has ceased. In contrast, if it happened only once or twice, there is not yet a Chazakah on it, and no check is required to prove that it has stopped.

g. Although the Shulchan Aruch (YD 196:2) writes that if the Zavah saw only one day and stopped on the same day (which is exactly the case we started with in your original question) she must do a Hefsek Taharah with a Moch Dachuk (which remains in place for the entire period of Bein ha'Shemashos), he is not referring to the Din in the time of the Gemara. Rather, he is referring to the Din nowadays, when all women have the status of Safek Nidah, Safek Zavah. My proof for this is that the source for the Shulchan Aruch is the Rashba cited here in the Beis Yosef. The Rashba writes explicitly that he is referring to contemporary women who are Safek Nidos, Safek Zovos. This is the same Rashba we cited above (in c.), who says that a Zavah Ketanah does not require a Hefsek.

I must stress that all of what we have been saying is l'Halachah v'Lo l'Ma'aseh, and certainly not to be relied upon for any practical purposes, especially since it is probable that there are other Shitos who disagree with the Shitos we have cited.

Kol Tuv,

Dovid Bloom