Siman 574

One who goes from a place where they are fasting to a place where they are not fasting, if he was not in his city at all at the time they accepted the fast upon themselves—the fast does not take effect upon him at all. And in this regard, "the stringencies of the place from which he departed" does not apply, for they do not decree upon one who is not in the city (see Taz). But if he was in his city at the time they decreed the fast, and he also accepted the fast upon himself—he is obligated to fast and complete it, even if he does not intend to return. However, if he was in the city and he did not explicitly accept the fast upon himself, in this it depends: if he intends to return—behold he is included among them, and they place upon him the stringencies of the place from which he departed, and he is obligated to fast and complete it. And if he does not intend to return—the fast does not take effect upon him, as with all matters where they do not place upon him the stringencies of the place from which he departed when he does not intend to return. (And with this the words of Rashi and Tosafot in Taanit 10a are clarified. For Rashi, in the comment beginning "to a place," explained: because of the stringencies of the place. And Tosafot, in the comment beginning "One who goes," explained: even if he does not intend to return, see there. And they do not disagree, as I have written, see there. Examine it closely and you will find it simple.)

One who goes from a place where they do not fast to a place where they do fast, and arrived there on the day of the fast, according to the law, if his intention is to return – the fast does not take effect upon him at all. Nevertheless, it is forbidden for him to eat while he is there, so that he does not separate himself from the community. Nevertheless, since he did not accept the fast upon himself, if he left the city beyond the techum, he is permitted to eat, and he does not have to complete it. But within the city and its techum – it is forbidden for him to eat even in private, because he must participate in the distress of the community. However, if the distress has passed – he is permitted to eat in private. And if his intention is not to return in any case – the fast takes effect upon him even outside the techum, and even if the distress has passed, according to the law of the stringencies of the place to which he went.

And all this is when he only arrived here on the day of the fast, and not before then at the time of the acceptance of the fast. But if he arrived beforehand, and was there at the time they accepted the fast—he is obligated to accept the fast upon himself, and he is obligated to complete it, so as not to separate from the community. And consequently, then there is no distinction between being in the city or outside the techum, whether his intention is to return or his intention is not to return, since he accepted the fast upon himself. However, if he did not accept it upon himself, and his intention is to return—the fast does not take effect upon him, and it is only forbidden to eat in their presence, meaning in the city and its techum. And in a place where he should not eat due to participation with the community and not according to the law, and he forgot and ate—he should not appear before them as if he ate, and he should not conduct himself with luxuries saying "since I have eaten a little I will eat a lot." Rather, he must stop immediately when he remembers. And even outside the techum he should not conduct himself with luxuries, but should only eat for the sustenance of the soul. And he should not conduct himself with the pleasures of eating and drinking and other pleasures (see Magen Avraham subsection 4).

It is a mitzvah for a person to starve himself in a year of famine, in order to participate in the distress of the community. And so too in any distress—it is a mitzvah to participate with the community. And if he is able to save them—he is obligated to save them, and if not—he should participate in their distress. And only if they stand in a place of danger—he is not obligated to also place himself in a place of danger, if there is no benefit to the community in this (Magen Avraham subsection 6). And one should not engage in marital relations in years of famine, except for the night of immersion. And one who has no children, and has not yet fulfilled "be fruitful and multiply"—it is permitted. And there are those who say that this law is not by strict law but rather a measure of piety (Tosafot 11a, s.v. "asur"). Therefore, if he may come to transgression—he should engage in relations. And nowadays, years of famine are not common (see Taz and Magen Avraham and Sha'arei Teshuva).