Siman 418

Rosh Chodesh is forbidden for fasting, as it is stated in the Yerushalmi (chapter 2, halacha 12) of Taanit: One may fast on all days, except for Shabbatot, Yamim Tovim, Rosh Chodesh, Chol HaMoed, Chanukah, and Purim. And in the Gemara Taanit (17b) it says regarding Megillat Taanit that Rosh Chodesh is from the Torah. And there are those who support this from that which is written: "And on the day of your gladness, and on your appointed festivals, and on your Roshei Chodashim," for it is juxtaposed to a festival. And Rosh Chodesh itself is called "Moed," as it is written: "He has called a Moed against me to crush my young men," and this refers to Rosh Chodesh Av. Nevertheless, our teacher, the Beit Yosef, wrote in his great book that it is not logical to say that the prohibition to fast is literally from the Torah; rather, it is very stringent, and there are supports for this from the Torah, but it is not literally de-oraita, see there. And also there is no obligation to eat bread, but only that one should not fast. Therefore, one who forgets "Ya'aleh V'Yavo" on Rosh Chodesh in Birkat HaMazon – we do not require him to repeat it.

One does not decree fasts upon the community for any trouble that may come on Rosh Chodesh, on Chanukah, or Purim, to begin the fasts on them. But if they began beforehand, such as if they previously decreed to fast such and such days and they began them before Rosh Chodesh, and at the time they decreed they did not say "except for Rosh Chodesh"; therefore, if they began even one day—they do not interrupt, and they fast and complete it, because a decree of the beit din and the community is very stringent. And it is like a complete vow, and this is not a literal Torah commandment such that a vow would not take effect upon it. Furthermore: vows take effect upon a matter of a mitzvah. And this is regarding a community. But an individual who accepted upon himself to fast such and such days, and Rosh Chodesh happened upon him, or who accepted upon himself to fast on Rosh Chodesh; if he accepted it in the language of a mere acceptance of a fast—it does not require annulment. For this is not like a complete vow, as I wrote in Yoreh Deah in siman 215. And if he accepted it explicitly with a vow, or if he said "it is upon me," which is like a vow—it requires the annulment of a sage. And in Yoreh Deah there, this was explained.

If one swore to fast for such and such days, and Rosh Chodesh occurred during them – the oath certainly takes effect upon him, even if it is from the Torah, by the principle of inclusion. And if he swore explicitly regarding Rosh Chodesh to fast upon it, our teacher, the Beit Yosef, wrote in section 3, and this is his wording: It appears to me that the oath takes effect upon him, since it is only from the Rabbis. However, from the words of the Rambam it appears ostensibly that it is from the Torah. Thus far his words. And in siman 570 this will be explained, see there. And one who fasts on Rosh Chodesh or on Chanukah and Purim a fast for a dream – he must sit for a fast for his fast.