Gemara

(Beraisa - R. Yoshiyah): "V'Hizartem... mi'Tum'asam" forbids a man to his wife shortly before her Veses (when she is due to menstruate).

Answer (Rabah): He must separate one Onah (a don).

Nidah 63b (Mishnah - R. Yosi): A Veses can depend on days and hours. If she normally sees at sunrise, she is forbidden only at sunrise;

R. Yehudah says, she is permitted the entire day (but she is forbidden the night before).

(Beraisa): If she normally sees on the 20th (of the month) during the sixth hour, when the 20th comes, if she did not see, R. Yehudah forbids relations during the first six hours; R. Yosi permits relations before the sixth hour, she refrains during the sixth hour.

If the sixth hour passed and she did not see, R. Yehudah forbids relations the rest of the day. R. Yosi permits from Minchah (after midday) and onwards.

(Beraisa #1 - R. Yehudah): She is forbidden before her Veses, she is permitted after it.

Contradiction (Beraisa #2 - R. Yehudah): She is forbidden after her Veses, she is permitted before it.

Resolution: In Beraisa #1, she normally sees at the end of the night; in Beraisa #2, she normally sees at the beginning of the day.

(Rava): The Halachah follows R. Yehudah.

65a (Beraisa - Beis Hillel): If a virgin saw blood before she was married, we give her (attribute all blood to Dam Besulim), a full Onah.

(R. Shimon ben Gamliel): An Onah is a night and half a day.

Question: Regarding Taharah of a winepress, R. Chiya bar Aba cited R. Yochanan to say that an Onah is a day or a night, and Rabah bar bar Chanah cited him to say that it is half a day and half a night;

(Rav Shmuel bar Rav Yitzchak): Both refer to 12 hours. R. Chiya discusses Nisan and Tishrei (when the nights and days are equal), and Rabah discusses winter or summer.

Answer #1: R. Shimon ben Gamliel means that an Onah is a day or night of Nisan or Tishrei, or half a day and half a night in winter or summer.

Answer #2: Besulah is different, for every wedding is delayed until they agree on (stipulations in) the Kesuvah and sign it (we assume that half the night will pass before they have relations).

Yevamos 62b (R. Yehoshua ben Levi): A man must be with his wife before he goes on a trip - "v'Yadata Ki Shalom Ohalecha (u'Fakadta Navcha)".

(Rav Yosef): This applies evens needed for Samuch l'Vestah.

Rishonim

Rambam (Hilchos Isurei Bi'ah 4:12): One may not have Bi'ah with his wife shortly before her Veses, lest she see blood during Bi'ah - "v'Hizartem..."

Rosh (Yevamos 6:10): We cannot say that he has Bi'ah with her Samuch l'Vestah, for the Torah forbids this! R. Tam explains that he speaks (intimately) with her. Most texts say that this is when she is Nidah (surely, we do not permit Bi'ah)!

Rosh (Nidah 9:2): If a woman normally sees at Netz, and she is not sure if it is before or after, the Ra'avad says that some are stringent and forbid the entire night and the entire day. Some are lenient, for Vestos are mid'Rabanan, and all the more so shortly before her Veses is mid'Rabanan, so we are lenient about a Safek, and we forbid only the day, which is Vadai within the days of (when she is expected to be) Nidah. The Ra'avad concluded like the lenient opinion.

Ran (Shevuos 1b DH v'Hizartem): "V'Hizartem" is an Asmachta, for we hold that Vestos are mid'Rabanan (Nidah 16a). Also, we permit one leaving on a trup to be Poked (have Bi'ah with) his wife shortly before her Veses. If it were mid'Oraisa, we could not permit it!

Poskim

Shulchan Aruch (YD 184:2): At the time of a woman's Veses, one must separate from her one Onah, i.e. from relations. If the Veses is during the day, he separates that entire day, even if the Veses is at the end of the day, and he is permitted immediately the coming night. If the Veses is at the beginning of the day, he separates that entire day, and he is permitted the entire previous night .If the Veses is during the night, he separates that entire night, and he is permitted the entire previous day and the next day. This is whether she established the Veses through one appearance or three.

Beis Yosef (DH uv'Sha'as): The Ran, Rashba and Rosh say that the verse is an Asmachta, for Vestos are mid'Rabanan. Sefer ha'Terumos and Tosfos and the Rosh in Yevamos say that Vestos are mid'Oraisa Perhaps they only mean that there is an Asmachta from the Torah.

Beis Yosef (DH u'Mah she'Chosav Im): All Poskim rule like R. Yehudah, who forbids only the Onah in which she normally sees, except for the Or Zaru'a. He obligates separating that Onah and the previous one, i.e. 24 hours in all. If she normally sees during the day, he separates that day and the previous night, and correspondingly if she normally sees at night. There is no reason or source for this. The Gemara explicitly says like the Poskim. The Agur (1358) calls the Or Zaru'a 'an excessive stringency.'

Bach (DH v'Nir'eh): The Or Zaru'a means that even though the Halachah follows R. Yehudah, it is proper to be stringent like the simple reading of R. Yirmeyah (our text - R. Yoshiyah), which forbids also the previous Onah, lest she see at the end of the night and think that it was the beginning of the day, or vice-versa. A support for this is the opinion that is stringent when she is unsure if she saw before or after sunrise.

Note: If she sees at the beginning of the day, but thinks that it is the end of the night, even if we forbid also the Onah before the Veses, due to her mistake she may have Bi'ah during her Veses!

Taz (2): It is astounding to be stringent. The Gemara is explicitly lenient!

Shach (7 DH uv'Or): Ra'avan explains that the Gemara discusses one who always sees at the same time within an Onah, but if she sees at varying times during the day, the entire day is her Veses, and surely she must refrain also the previous Onah! Or Zaru'a discusses such women. Also Sha'arei Dura and the Maharil bring the Or Zaru'a l'Halachah.

Beis Yosef (DH Kasvu Hagahos): Hagahos Maimoniyos (4:9) says that Onah is a day or night in Nisan or Tishrei, or half of a day and night in Tamuz or Teves. He holds that Onah is 12 hours. I say that one need not be concerned for this, since the Poskim did not mention it. Also the Gemara connotes that we are concerned only for a day or night, and not for hours. It mentioned hours in Nidah only due to R. Yosi's opinion.

Rebuttal (Bach DH Kasav): Nidah 65a says that Onah is 12 hours. The Halachah follows this.

Support (of Bach - Shach 7): Also Ma'adanei Melech (Nidah 1:2) and Ra'avan say so. The Poskim did not mention this because they discuss an average day or night, but indeed, in winter or summer we must calculate relative hours, like we find regarding Kri'as Shema, Chametz on Erev Pesach, and calculations of Tekufos (seasons).

Hagahos Chasam Sofer: These words are astounding.

Sidrei Taharah (3 DH Mah): The text of the Shach should say equal hours. He must hold that the times for Kri'as Shema and Chametz are three and four equal hours; this equals three or four relative hours only in Nisan or Tishrei, and likewise regarding Onah. I say that Aviasaf learned from Nidah 63b. Day begins at dawn. Why does the Gemara discuss sunrise? In Nisan or Tishrei, there are 12 hours from sunrise to sunset. Between dawn and sunrise is considered part of the previous Onah.

Defense (of Beis Yosef - Taz 2): If it depends on hours, the Onah in which she sees is a full day or night only in Nisan or Tishrei, and only if she sees at midday or midnight. If she sees at the end of the day, if we count the 12 hours from that time, she is permitted during the day before this time, and forbidden the next night. R. Yehudah and all the Poskim forbid her during the day! If the 12 hours end at the time she sees, if she sees at the start of the day, she is permitted during the day after this time. Chas v'Shalom to say so! Even if we forbid six hours before and six after, she is not forbidden the entire day. Rather, it depends on the day or night, whether it is long or short, whether she sees at the beginning or end. Chachamim gave a fixed Shi'ur. Nidah 65 discusses Dam Besulim. It depends on hours, and not on day or night. It is unreasonable to give only a short time to one who had Bi'ah at the end of the night, and to give the entire night to to one who had Bi'ah at the beginning of the night. Vestos depend on when she normally sees, during the day or at night.