Commentary for Nedarim 120:6

מהו דתימא ראש חדש לשעבר הוי ולא ליתסר קמ"ל קרו אינשי ריש ירחא:

'THIS MONTH,' HE IS FORBIDDEN THE WHOLE OF THAT MONTH, BUT THE BEGINNING OF THE [FOLLOWING] MONTH BELONGS TO THE FUTURE. This is obvious? — It is necessary only when the [following] Month is defective: I might think that the new Moon belongs to the past, and is forbidden: it is therefore intimated that people call it new moon.<span class="x" onmousemove="('comment',' The months of the Jewish year consist of either twenty-nine or thirty days and generally alternate. Hence, if the following month is detective (i.e., of twenty-nine days), this one is full. In the month following a full one, the first two days are designated 'new moon', the first being really the thirtieth day of the past full month. Hence, if one vowed in a full month, it might be thought that he is bound on the first new moon day of the next. Therefore the Mishnah teaches that since it is called new moon, People generally regard it as part of the next month, and hence he is permitted thereon. — This is the reading of Asheri, Ran and Tosaf. But our editions, and Rashi too. have: I might think that the new moon belongs to the past, and should not be forbidden. This reading cannot be reconciled with the first interpretation of the Mishnah, but agrees with the second (q.v. p. 190, n. 5). If he vowed 'this month' on the first new moon day, I might think that since it actually belongs to the past month he is not forbidden thereon. Therefore it is taught that since it is designated new moon, he must have meant to include it. ');">⁵</span>

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