Nedarim, Chapter Ten, Mishnah Eight
Numbers 30:6, 9 imply that the husband or father must annul their wives or daughters vows on the day on which they heard of them. Our mishnah explains how this day is calculated.
The annulment of vows is the whole day.
This may result in a stringency or in a leniency.
How is this so?
If she vowed on the eve of the Sabbath, he can annul on the eve of the sabbath and on the Sabbath day until nightfall.
If she vowed just before nightfall, he can annul only until nightfall: for if night fell and he had not annulled it, he can no longer annul it.
The period allotted for annulling vows is one whole day. That day is calculated by the calendar and not by counting 24 hours from the time the father/husband heard the vow. In other words, the vow must be annulled on the calendar day on which it was taken. Remember, the Jewish day is calculated from evening until evening. As the mishnah points out, sometimes this will result in a longer time and sometimes in a shorter time. For instance, if he hears of the vow on the eve of the Sabbath (late Friday afternoon, towards twilight), he has until nightfall the following night, when the Sabbath is over. This is because twilight is already considered part of the Sabbath and the Sabbath is not over until nightfall.
However, if he hears of the vow just before nightfall on the Sabbath itself, he only has until nightfall. After that he may no longer annul the vow.
The reason that the mishnah uses the example of the Sabbath and not any other day of the week is to teach that vows may be annulled on the Sabbath.