HAFARAH MUST BE ON YOM SHOM'O [Nedarim: Hafarah: Yom Shom'o]
(Mishnah): A vow may be annulled on the day it is heard (by the father or husband). This can be a leniency or a stringency:
If she vowed on Shabbos night, he can annul on Shabbos night, and Shabbos day until dark. If she vowed shortly before nightfall, he can annul only until nightfall.
(Beraisa): Vows can be annulled the entire day;
R. Yosi b'Rebbi Yehudah and R. Eliezer b'Rebbi Shimon allow 24 hours.
(R. Yehoshua ben Levi): The Halachah does not follow that pair.
Levi was about to rule like R. Yosi and R. Eliezer.
Rav: R. Chiya said that the Halachah does not follow them.
Chiya bar Rav would throw arrows. Rabah bar Rav Huna would sit and stand (this will be explained).
78b (R. Chanina): A husband who is silent to vex his wife (so she will not know that he will annul later) can annul even 10 days later.
79a - Question (Rav Kahana - Beraisa): "If her husband will be silent (her vow is affirmed)" discusses silence with intent to vex.
The end of the verse "for he was silent" teaches silence with intent to affirm, so the Reisha must discuss silence with intent to vex.
This refutes R. Chanina.
Shabbos 157a (Mishnah): We may annul vows on Shabbos, and Nish'alim (we request Beis Din to permit them), if they are for the need of Shabbos.
Question: Is Hafarah even not for a need of Shabbos (because Hafarah is only until nightfall, and cannot be done after Shabbos)?
If so, we understand why Hafarah and Nish'alim were taught separately.
Or, perhaps also Hafarah is only if needed for Shabbos. They were taught separately because Hatarah requires a Beis Din, but Hafarah does not!
Answer (Zuti d'Vei Rav Papa's Beraisa): We may annul for the need of Shabbos.
Inference: If it is not for the need of Shabbos, it is forbidden. This teaches that one has 24 hours to annul.
Contradiction (Rav Ashi - Mishnah): Hafarah is all day (until nightfall...) If he did not annul it before dark, he can never annul it.
Answer: Tana'im argue about this (in Beraisa (2) above).
The Rif brings the Gemara (76b) verbatim.
Rif and Rosh (10:11): The Halachah does not follow R. Chanina, for he was refuted.
Rif (Shabbos 68a): We concluded that Tana'im argue about whether vows can be annulled the entire day, or for 24 hours. The Halachah is, they may be annulled only the entire day.
Rambam (Hilchos Nedarim 12:15): Vows can be annulled the entire day. We do not allow 24 hours. If she vowed on Sunday night, he can annul that night, and Yom Sheni (Monday during the day). If she vowed Monday morning, he can annul the entire day (during the day). If she vowed shortly before dark, he can annul only before dark.
Rambam (18): If her father or husband was silent to pain her, even though he did not intend to affirm, since he did not annul that day, her vows are affirmed.
Rosh (10:7): R. Yehoshua ben Levi and Rav ruled that the Halachah does not follow that pair, i.e. R. Yosi and R. Shimon. Some texts say 'the Halachah follows that pair', because in Shabbos we said 'the Halachah is, we may annul vows for 24 hours.' These texts explain that Levi planed to rule like these Tana'im, i.e. the Tana of our Mishnah and the first Tana of the Beraisa. Even if the text says 'the Halachah does not follow that pair', we can say that the Stam Gemara (in Shabbos, which rules like the pair) argues with R. Yehoshua ben Levi and Rav. According to the text 'the Halachah follows that pair', we can explain that Chiya bar Rav would throw arrows and Rabah bar Rav Huna would sit and stand when their wives vowed and they wanted to vex them. They would delay almost 24 hours before annulling it. They would not annul until the next day when the (arrow's or his) shadow reached almost the same place as at the time of the vow. If the text says 'the Halachah does not follow that pair', we must say that this practice of Chiya and Rabah does not belong here. (They engaged in other activities while permitting vows because they hold that regret is sufficient, so there is no need for great concentration.) The text in Shabbos of R. Chananel and the Rif does not say 'the Halachah is, we may annul vows for 24 hours.' One should be stringent like them.
Shulchan Aruch (YD 234:21): A man can annul the vows of his daughter or wife only on the day that he heard them, not all 24 hours. If she vowed at the beginning of the night, he can annul that night, and the next day. If she vowed shortly before dark, he can annul only before dark. If the day passed and he was silent, even if he did so to pain her, he cannot annul.