Gemara

(Mishnah - R. Yosi): If the Korban Pesach was offered for a Shomeres Yom k'Neged Yom (a woman on the morrow of seeing non-menstrual blood), and later that day she saw blood, she is exempt from Pesach Sheni.

R. Yosi holds that Miktzas ha'Yom k'Kulo. (The very start of the day after her first sighting is like a full clean day, so her next sighting is not Metamei her retroactively.)

Question: According to R. Yosi, how can a woman bring a Korban for Zivah [for seeing blood three consecutive days]? The first part of the second [or third) day, before she saw, should be considered an intervening clean day!

Answer #1: Blood flowed continuously from the first day until the third day.

Answer #2: On two consecutive days, she saw all of Bein ha'Shemashos.

Megilah 20a: A Shomeres Yom cannot immerse the next night. This is because she must count (a clean day), and counting must be by day.

Mo'ed Katan 17b (Beraisa): Chachamim agree that if an Avel's eighth day is on Shabbos that is Erev ha'Regel, he may shave on Erev Shabbos.

Rishonim

Drashos Ri ibn Shu'iv (Yom Rishon of Pesach, DH v'Lachen): Therefore, the custom is to grow Safam (moustache) [from Pesach] until Shavu'os, and not to marry women then (previously, he mentioned that the Omer offering is like Minchas Sotah - PF). [This is primary,] even though the Midrash gives another reason, i.e. the 12,000 pairs of Talmidim of R. Akiva died between Pesach and Shavu'os. Most people grow Safam until LaG b'Omer. I know no reason for this. Tosfos says that LaG is not like the custom, rather, we remove [from the Omer] the seven days of Pesach, the seven Shabbosos, and two days of Rosh Chodesh, which are 16 days on which we do not mourn.

Bach (OC 493:1): One of the Shabbosos is during the seven days of Pesach! Rosh Chodesh Sivan is the 16th day. (He assumes that Ri ibn Shu'individual counted the two days of Rosh Chodesh Iyar.)

Ri ibn Shu'iv: LaG (33) days remain. A Midrash says "until Pras (15 days before) Shavu'os, like we discuss Pras (15 days before) Pesach and Sukos. Thirty-four days remain. We observe 33 full days, and shave on the 34th in the morning, for Miktzas ha'Yom k'Kulo.

Tashbatz (1:178): The Gemara does not mention the Isur of Nisu'in during the Omer. It says only that Talmidei R. Akiva died because they did not show honor to each other (Yevamos 62b). It does not say what was the lack of honor. We did learn that your colleague's honor should be dear to you like your own, and like the fear of your Rebbi (Avos 2:8, 4:13). They did not do so. They caused Torah to be profaned. People said "woe to one who learned Torah!" The custom is to forbid only until LaG b'Omer. This is astounding. Ba'al ha'Ma'or found in an old Sefardi Sefer that they died between Pesach and Pras Shavu'os. Pras is not less than 16 days. (It seems that this should say "15", like he says below - PF.) This shows that Pras is half; we learn Hilchos Pesach 30 days beforehand, and half [of the 30 days before Shavu'os] is 15. [We forbid until] LaG b'Omer, and Miktzas ha'Yom, i.e. part of the day of the 34th, k'Kulo, like all laws of Avelus. Therefore, the entire day of the 33rd is forbidden, and part of the 34th. The Ramban in Toras ha'Adam holds that part of the night suffices. If the 33rd is Erev Shabbos, we permit [cutting hair] that day for Kevod Shabbos, just like we permit an Avel within Sheloshim (30 days from death) to cut hair on Erev ha'Regel, for Kevod ha'Regel. The same applies to Kevod Shabbos. Similarly, if an Avel's eighth day is on Shabbos (Erev ha'Regel), he may shave [on day seven], even though if it fell on a weekday he would be forbidden unless there were seven days before the Regel. He mentioned LaG, and not 34, because the 34th day is not complete.

Poskim

Shulchan Aruch (OC 493:2): The custom is not to cut hair until LaG b'Omer, for they say that then [R. Akiva's Talmidim] ceased dying. One should not cut hair until the morning of the 34th, unless the 33rd day is Erev Shabbos. Then, one may cut hair on the day for Kevod Shabbos.

Beis Yosef (DH u'Mah (2)): The Avudraham brings from Even ha'Yarchi, citing Ba'al ha'Ma'or, that we cut hair from LaG b'Omer and onwards. Our custom is like Ri ibn Shu'iv says. Also R. Yerucham says so.

Gra (DH Nohagim): This is like we say in YD 395:1, that Miktzas ha'Yom k'Kulo, [i.e. we forbid only] until sunrise. They bring a proof from Megilah 20a.

Mishnah Berurah (7): Even though they died also on the 34th, this is why we are stringent the entire night of the 34th until sunrise. Perhaps they mostly died by the 33rd, and only a few died on the 34th.

Kaf ha'Chayim (18): The Pri Chodosh says that one who relies on the opinion that applies Miktzas ha'Yom k'Kulo at night does not lose. However, we accepted the Mechaber's rulings, so one should not be lenient.

Kaf ha'Chayim (13): The Ari Zal shaved only on Erev Pesach and Erev Shavu'os, but not on Rosh Chodesh Iyar or LaG b'Omer. One may not shave on the 48th day, rather, only on the 49th. Nehar Shalom was stringent even for a Chasan. Igros ha'Ramaz says that the Ari Zal is stringent even if a Bris Milah occurs. Nehar Shalom was unsure if the 49th day was Shabbos, if the 48th is permitted. The Birkei Yosef says that he should shave, lest he enter Shavu'os unkempt.

Bi'ur Halachah (DH Nohagim): Those who may cut hair during Chol ha'Mo'ed (Siman 531) are permitted during Sefirah. It is no more stringent than Chol ha'Mo'ed.

Rema: Here, this is not the custom. Rather, we cut hair on the 33rd, and have some extra Simchah. One cuts on the 33rd itself, but not the night before.

Magen Avraham (3): Kavanos [ha'Ari Zal] says that a certain Gadol used to say Nachem (the supplications over the Churban that we add in Boneh Yerushalayim in Minchah on Tish'ah b'Av - PF) every day, and he said it also on LaG b'Omer, and he was punished.

Gra (DH u'MArbim): This is like it says in Ta'anis (30b), that Av 15 was a Yom Tov because that is when they ceased dying in the Midbar.

Mishnah Berurah (8): This opinion holds that they totally stopped dying on the 33rd.

Mishnah Berurah (9): We do not say Tachanun also at Minchah the day before.

Rema (ibid.): However, if the 33rd is Sunday, the custom is to cut hair on Erev Shabbos for Kevod Shabbos.

Mishnah Berurah (11): Some Acharonim are lenient to cut hair the night before. Eliyahu Rabah concludes "in any case, I did not see people lenient about Nisu'in at night, only on the 33rd day itself. However, if LaG b'Omer is on Erev Shabbos, and it is difficult to have the Nisu'in that day, one may be lenient to make it the night before."

Kaf ha'Chayim (23): If he will not be able to shave during the day, he may shave the night before.

Shulchan Aruch (3): Some have the custom to shave on Rosh Chodesh Iyar. This is a mistake.

Beis Yosef (DH v'Yesh Omrim): Rosh Chodesh is like Yom Tov, so one should not conduct Avelus. Since most do not conduct like this, I say that it is a mistake. They learned from Ri ibn Shu'iv, who says that Tosfos excludes the two days of Rosh Chodesh. This is only for those who forbid until Shavu'os! We forbid until LaG b'Omer, therefore all 33 days are forbidden.

Kaf ha'Chayim (40): Maharam Lunzanu and Maharikash say that it is a proper custom.

Mishnah Berurah (14): One opinion permits the first 16 days, and forbids every day from the second day of Rosh Chodesh Iyar, except for LaG b'Omer, to which we apply Miktzas ha'Yom k'Kulo.

Rema: In many places they shave until Rosh Chodesh Iyar. They may not shave after LaG b'Omer, but they may shave on LaG b'Omer itself. Where they shave after LaG b'Omer, they may not shave after Pesach until LaG b'Omer.

Taz (2): This opinion holds that they died also after LaG b'Omer. LaG is a mere Siman that they died on 33 days. If so, there is no reason to increase Simchah on LaG b'Omer. The custom everywhere is to have Nisu'in on LaG b'Omer, but not before or after. There is no reason for this. I say that the primary opinion is that they died only until LaG b'Omer. We conduct a little Avelus afterwards due to the decrees of 4856 (the first Crusade), which were in Ashkenaz between Pesach and Shavu'os. We are stringent only about Nisu'in after LaG b'Omer, but one may shave.

Magen Avraham (5): Here, the custom is to marry and shave in the three days before Shavu'os. Therefore, we forbid these on Rosh Chodesh Iyar, and the start of the first of the three days before Shavu'os, so Miktzas ha'Yom k'Kulo. Another opinion permits the seven days of Pesach, six Shabbosos, and two days of Rosh Chodesh Iyar and one day of Rosh Chodesh Sivan; 33 days remain. The Rema omitted this, for he disagrees.

Mishnah Berurah (15): Everyone should adopt the custom of his place.

Rema (ibid.): In one city, there may not be different customs, due to Lo Sisgodedu. All the more so, one may not adopt both leniencies!

Magen Avraham (6): If most of the Tzibur conducts one way, the minority must do like them.

Mishnah Berurah (16): One who adopts both stringencies, and permits only LaG b'Omer and one or three days before Shavu'os is not "a fool walking in darkness", since he does so because he is unsure which custom is primary. One need not be so stringent, for it is a mere custom. If one knows the custom of the place, he should not deviate at all.