We have learned that the ability of a woman’s father or husband to nullify her neder – hafarat ha-neder – is limited to the day on which he heard that the vow had been taken (see Bamidbar 30:15-16). The Mishna on our daf teaches that the vow can be nullified “all day.” What is less clear is how to define the day. A baraita brought by the Gemara has the Tanna Kamma permitting hafara until nightfall of the day, while Rabbi Yosei and Rabbi Elazar ben Shimon rule that there is a 24 hour period after the husband or father hears of the neder, during which he can be mefer.
The example brought by the Mishna is a case where the woman took her neder on Friday night. Following the opinion of the Tanna Kamma, the man has all night Friday and throughout Shabbat to be mefer. Once Shabbat is over, the opportunity for hafara is lost.
It is clear that the case of Shabbat is merely an example; the same halakha would be true if the neder was taken on a normal weekday. The example of Shabbat is used to clarify the rule that the day is defined by the normal halakhic day, where the night is seen as a single unit with the day that follows it and not with the day that precedes it. Tosafot ha-Hadashim point out that it is particularly important to emphasize this halakha with regard to nedarim, since – as we have learned previously – many of the rules of nedarim parallel the rules of kodashim – of objects that are consecrated to the Temple. The rules of kodashim are unique in that the night is considered part of the previous day in many cases, and it is essential to clarify that nedarim do not follow this pattern.
In his commentary to the Mishna, the Rambam offers an alternative reason for the choice of Shabbat as an example of hafarat nedarim. The Rambam explains that the Mishna takes the opportunity to teach an additional halakha that might not be obvious – that hafarat nedarim can be done on Shabbat.