Shulchan Arukh Orach Chayim · Siman 574

English

1. (The Laws) that we give in matters of fasting the stringency of the place of those leaving and those coming to there, and in it is 5 paragraphs. One who is going from a place that is fasting to a place that is not fasting, and his intention is to return, he needs to fast the whole fast that was accepted upon him (via the members of the fasting city).

English

2. One who is going from a place that is not fasting to a place that is fasting, he should fast with them even if his intention is to return. However, since he did not accept upon him, if he leaves the city, outside of the Halachic Border, it is permitted to eat and it is not necessary to complete (the fast). Note (Rema): Specificially, if he came on the day of the fast, but if he came when it was still day (that is prior to the fast beginning), it is necessary to accept the fast upon himself and he needs to complete it [Tur]

English

3. If he forgot himself and ate and drank (on a day observed as a fast day in the place), he must not show himself before the public, nor must he indulge himself with delicacies, saying since I ate a little, I'll eat a lot. Parentheses (Rema): even not in front them, it is forbidden to accustom himself with delicacies [Mordechai, First Chapter of Taanis]

English

4. It is a mitzvah to famish oneself (i.e. eat little) in a year of famine, and it forbidden to have marital relations except for the Night of Immersion and one who lacks children explanation: to those who have been deprived of children it is allowed. Parentheses (Rema): See prior in Siman 240 Paragraph 12.

English

5. Anyone that separates from the community (in times of distress), he will not see its consolation, and anyone that grieves with them, he will merit and see it in its consolation.