Siman 572

From the law of the Gemara, one does not decree fasts initially upon the public on a Thursday, so as not to increase the market prices. When they see that people are purchasing two meals, for the needs of the night and for the needs of the Shabbat, they will think that a famine has come to the world, and they will increase the price of grain. And even in a place where there is no concern for this – one should not do so. Nevertheless, nowadays, when price gougers are not common, they wrote that it is permitted to decree a fast on a Thursday. And such is the custom (Magen Avraham, subsection 2).

We do not decree fasts upon the community on Roshei Chodashim, on Chanukah and Purim, or on Chol HaMoed. And if they began to fast over a distress even for one day, and one of these days occurred among them – they fast and complete the day in fasting. And even though Rosh Chodesh is called a "Moed," nevertheless it is not a day of feasting and joy. And this is regarding the community. But regarding an individual fast – one interrupts on these days. And nowadays, all fasts that the community decrees – they are like an individual fast, and they interrupt on Rosh Chodesh and complete another day (Beit Yosef and Magen Avraham subsection 4). And such is the custom (Bach).

It appears to me that regarding a fast of an individual, when he accepted upon himself to fast such and such days, he also interrupts on other days in which Tachanun is not recited. For since these days are established among us as days on which one should not fast—certainly his intention was also thus, unless he specified them explicitly. And so too nowadays regarding a community, whose law is like an individual in this era as I have written—the law is also thus, when they did not specify explicitly to fast also on these days.

A community that sought to decree a fast of Monday, Thursday, and Monday, and they decreed so, and the first fast fell on the fifteenth of Shevat – they should not begin to fast on it, for certainly their intention was not for it. Rather, it should be postponed to the Monday or Thursday after it, because it is the Rosh Hashanah for trees, and it is not proper to begin a fast on it. However, if they began to fast – they do not stop for them, for it is no greater than Rosh Chodesh or Chol HaMoed. Therefore nowadays, since it is considered like a fast of an individual as I have written – they stop even though they began, and as I have written.

And all this is regarding a fast. However, an individual who accepted upon himself not to eat meat from today for such and such days—it is forbidden for him to eat even on Shabbat and Yom Tov, for there is no obligation to eat meat specifically, unless he says that his intention was not for Shabbats and Yamim Tovim. And a community that decreed in such a manner—it depends on the beit din, what their intention was (see Magen Avraham, subsection 5). And the same law applies to anything that the individual or the many accepted, such as wine and strong drink and fish and the like.