Depending on how it is phrased, the endpoint of a vow might be ambiguous. For instance:
Wine is forbidden for me and I will not taste it until it will be Passover.
If the person making the vow had simply said, “until Passover,” it would be a clear statement that the holiday is not included in the vow. Or, had they said, “until Passover is over,” their intention to refrain from wine until the holiday ends would have been made clear. But interpreting the phrase “until it will be Passover” is difficult. You could say that the addition of “it will be” adds to the “until” and includes the holiday in the vow. Or, you could say it doesn’t change the meaning. Which is correct? Rabbi Yehuda is quoted in the mishnah as providing some direction:
Rabbi Yehuda says: In the case of one who says, “Wine is forbidden for me, and I will not taste it until it will be Passover,” it is understood that this individual intended for his vow to apply only until the night of Passover, that is, until the time when it is customary for people to drink wine.
One of the central components of the seder (Passover’s ritual meal) is drinking four cups of wine. As Rabbi Yehuda sees it, given this mandate, it’s safe to assume that the person intended to end their vow-imposed prohibition when the holiday starts.
Likewise, says the mishnah:
One who says “Meat is forbidden for me, and I will not taste it until it will be the fast of Yom Kippur,” — he is prohibited from eating meat only until the eve of the fast. This is because it is understood that this individual intended for his vow to apply only until the time when it is customary for people to eat meat.
As in the case of the vow to abstain from wine until Passover, in this case we assume that the vower intended to begin eating meat at the start of the holiday, as this was apparently a customary component of the pre-fast meal. Except in this case, the vower can’t eat meat on Yom Kippur — because it’s a fast day on which they do not plan to eat at all. Which means, Rabbi Yehuda infers, that they actually intended their vow to end just before the fast. So while in the Passover case the vow ends as the holiday begins, here it ends a little bit before the holiday. Same language, different meaning — because of the context.
We’ve seen throughout this tractate that the rabbis wish for language to be precise. But we also see here that there is an understanding that the same words can mean different things in different contexts and, for the rabbis, colloquial use is taken into account in legal rulings. The Talmud doesn’t have more to say on this because there is no Gemara on this mishnah.
Incidentally, there is a third component of the mishnah in which Rabbi Yehuda’s son states that one who has vowed not to eat garlic “until it will be Shabbat” ends this vow in time to eat a garlicky Shabbat dinner on Friday night which, as we were reminded back on Nedarim 31, was popular. Still is!
Read all of Nedarim 63 on Sefaria.