Siman 573

Among the early Tannaim there was "Megillat Taanit," which were the days that the Holy One, Blessed be He, performed miracles for Israel, and those days were forbidden for eulogy and fasting. And it was taught in Rosh Hashanah (19b): These days written in Megillat Taanit, whether in the time that the Beit HaMikdash exists, or in the time that the Beit HaMikdash does not exist—they are forbidden; these are the words of Rabbi Meir. Rabbi Yossi says: (...) In the time that the Beit HaMikdash does not exist—they are permitted. And the halacha is like Rabbi Yossi. And the Shas ruled there that the halacha is that Megillat Taanit is nullified, except for Chanukah and Purim, which are forbidden for fasting on the days themselves. But before them and after them is permitted in this time, even though in the time of the Mikdash they were forbidden. And all of Megillat Taanit—even on these days they are permitted, and all the more so before them and after them. And so too Sabbaths and Yamim Tovim, and Roshei Chodashim—it is permitted to fast both before them and after them. And this was so even in the time of the Mikdash. And the reason for the matter is this: that which they forbade in the time of the Mikdash regarding Megillat Taanit to fast before them and after them—was in order to strengthen the matter of the days themselves. And this applies to Rabbinic decrees which require strengthening, but Sabbath and Yamim Tovim and Rosh Chodesh, for which it is forbidden to fast by the Torah, anything from the Torah does not require strengthening. (Rosh Hashanah ibid. And there it is explained that Rosh Chodesh is from the Torah, see there. And see the end of siman 570, and it requires further study.)

Our teacher, the Rema, wrote: One who has a wedding during Chanukah—he should not fast. But if he has a wedding in Nisan—he fasts on the day of his chuppah, even on Rosh Chodesh Nisan, because it is one of the days on which they fast, as will be seen below at the end of siman 580. Thus far his words. And so too on Lag BaOmer, and during the three days of hagbalah, and between Yom Kippur and Sukkot—they fast, since it is only a custom. But on Isru Chag, and on the fifteenth of Av, and on the fifteenth of Shevat, since they were mentioned in the Gemara—they do not fast (Magen Avraham). And the reason for the fast of the chatan on the day of his chuppah was explained in Even HaEzer siman 61, see there.