Nedarim Chapter 8, Mishnah 6. In the previous mishnah we examined the phrase "until it will be": one who says that a certain item is forbidden to him until a known time - Pesach or another festival - means until that time has ended. This rule, however, can change when it is clear from the context that the person taking the vow intended an earlier point in time.
A further example of this in our mishnah:
"Konam basar she'eini to'em ad sheyehei hatzom - eino asur ela ad leilei hatzom" - one who forbids himself to eat meat until the fast: even though the phrase "until it will be" would seem to include the fast itself (and certainly, once the fast has begun there is no possibility of eating at all), he is forbidden only until the eve of the fast, toward evening before the fast day.
The reason: the mishnah is dealing with Yom Kippur, and it refers to the final meal before the fast, at which it was customary to eat meat. Since this was the practice, the intention of the one taking the vow is not Yom Kippur itself, but the time preceding it: "lo nitkaven zeh ela ad sha'ah shederech benei adam le'echol basar" - only until the hour at which people are accustomed to eat meat, which is the final meal before the fast.
The opinion of Rabbi Yosei son of Rabbi Yehudah:
Although Rabbi Yehudah himself is not mentioned in our mishnah, in the previous mishnah Rabbi Yehudah was mentioned as the one who stated this idea, and here come the words of his son: "Konam shum she'eini to'em ad haShabbos - eino asur ela ad leilei Shabbos" - one who forbids himself to taste garlic until Shabbos: even though his words would seem to mean until the end of Shabbos, he is forbidden only until Friday night.
And the reason: "lo nitkaven zeh ela ad sha'ah shederech benei adam le'echol shum" - only until the hour at which people are accustomed to eat garlic. The practice of eating garlic on Friday night is one of the ordinances of Ezra, for garlic is beneficial for procreation, and since people are accustomed to be with their wives on Friday night, the Sages ordained eating garlic in order to increase fertility. Therefore, even though he said "until Shabbos," he means only until Friday night, so that he will be able to eat garlic at the Friday night meal.
In summary: in this mishnah we learned that the phrase "until it will be" gives way to the clear intention of the one taking the vow: one who vows against meat until the fast is forbidden only until the eve of the fast, because of the custom of eating meat at the final meal before the fast; and one who vows against garlic until Shabbos is forbidden only until Friday night, because of Ezra's ordinance to eat garlic on Friday night. In both cases, the person taking the vow intends only until the hour at which people are accustomed to eat that food.