Siman 570

The Rambam wrote in the third chapter of Hilkhot Nedarim, law 9: One who vows to fast on Shabbat or on a Yom Tov is obligated to fast, for nedarim take effect upon a matter of a mitzvah. Similarly, one who vows to fast on Sunday or on Tuesday all his days, and this day happens to be a Yom Tov or Erev Yom Kippur—he is obligated to fast. And it is unnecessary to mention Rosh Chodesh. If Chanukah or Purim happen to fall on it—his neder is set aside because of these days; since the prohibition of fasting on them is from the words of the Scribes, they require strengthening, and it is set aside because of the decree of the sages. Thus far his words. However, the Tur in Yoreh Deah siman 215 ruled that the neder takes effect even upon them, and they release his neder for him. And this is when he accepted it with the language of a neder. But if he accepted it with the language of accepting a fast—it does not set aside Shabbat or Yom Tov, nor Chanukah or Purim, and he does not require inquiry at all, see there. And this is according to what we explained at the beginning of siman 562, that there is a distinction between a neder and a fast. This is also the opinion of the Ra'avad, and the Ramban, and the Rosh, and the Ran, as we explained there in Yoreh Deah in this siman, section 19, see there.

This is what our teacher, the Beit Yosef, wrote in section 1: An individual who accepted upon himself a fast of such and such days, and Sabbaths and Festivals occurred within them, or Rosh Chodesh, Chanukah and Purim, or the eve of Yom Kippur, if he accepted it with the language of a general acceptance of a fast – he does not require annulment. But if he accepted it with the language of "behold it is upon me," which is the language of a vow – he requires annulment by a sage. And he opens with regret, for he shall say to him: "If you had set to your heart that these days would occur within them, you would not have vowed," and he annuls it for him. And according to the Rambam, once he accepted it upon himself with the language of a vow, and Sabbaths and Festivals, or the eve of Yom Kippur, or Rosh Chodesh happened upon him – he is obligated to fast on them if they do not annul it for him. But if Chanukah and Purim happened upon him – his vow is void and he shall not fast on them, because they are from the words of the sages and require reinforcement. Thus far his words. The explanation of when he said "behold it is upon me" is: he must say "behold the eating of such a day is a konam upon me," or a vow or a prohibition. But if he said "behold it is upon me to fast" – it is not called a vow (Magen Avraham, subsection 2), for we require a prohibition on the object, and a fast is not an object. And it is like any acceptance of a fast.

The primary ruling is according to the opinion of the majority of the poskim. And one who vowed to fast such and such days and said "except for Yamim Tovim" – Hanukkah and Purim are not included, for they are not called "Yom Tov" in the common language of people. And if he said that his intention was also for them – he is believed. And the eve of Yom Kippur is included in "Yom Tov," and all the more so Rosh Hashanah and Chol HaMoed. But not Rosh Chodesh, unless he said that his intention was also for Rosh Chodesh.

One who vows to fast on Shabbat, and Yom Tov, and Erev Yom Kippur, and Chanukah, and Purim, and Rosh Chodesh—their law is the same as one who vows to fast such and such many days, and these days occurred within them. For if he expressed them in the language of a neder—the neder takes effect, and requires hatarah. But if he expressed them in the language of accepting a fast—the neder does not take effect, and does not require hatarah. (And see what I wrote in siman 604 regarding the matter of Erev Yom Kippur according to the opinion of the Rambam.)

It is a known matter that even though a neder takes effect upon a matter of a mitzvah, a shevuah does not take effect unless it is through inclusion, as I wrote in Yoreh Deah siman 215. Therefore, if one swore to fast for such and such days, and these days occurred within them, he is obligated to fast by reason of inclusion. But if he swore to fast on Shabbat or Yom Tov explicitly—the shevuah does not take effect upon him. It is a shevuat shav, and he receives lashes, and he eats. For a shevuah does not take effect upon a matter of a mitzvah. And if he swore to fast on Chanukah and Purim—the shevuah takes effect upon him, since they are only midrabbanan. And one should annul his shevuah, and he does not fast. The same law applies to Rosh Chodesh and Erev Yom Kippur—they too are only midrabbanan. However, from the Rambam it appears that they are from the Torah (and so it is implied in Rosh Hashanah 19a). And there is one who says that Rosh Chodesh is drabbanan, and Erev Yom Kippur is de'oraita (Magen Avraham subsection 9). And in both of them, one should annul his shevuah to fulfill all opinions.