Gemara

R. Yanai told his children "do not bury me in white clothing. Perhaps I will be in Gehinom, and I will be like a Chasan among mourners!"

Ta'anis 10b (Beraisa): If one went from a place where they do not fast to a place where they fast, he fasts with them. If one went from a place where they fast to a place where they do not fast, he fasts and completes the fast. If one forgot and ate and drank, he should not show off in front of the Tzibur, and he should not indulge himself in pleasures.

Pesachim 51a: When Rabah bar bar Chanah came to Bavel, he ate De'aisra (Chelev on the 'bow'. Bnei Eretz Yisrael eat it, but Bnei Bavel do not eat it.) Rav Avira Sava and Rabah bar Rav Huna came to see him. He covered it up.

Chulin 18b (Rav Ashi): Even if a resident of Bavel goes to Eretz Yisrael, if he does not intend to return, he adopts all the customs of his new locale.

110a: Rami bar Tamri went from Pumbadisa (where they eat udder) to Sura on Erev Yom Kipur, and saw everyone discarding udders. He gathered them and ate them.

Rav Chisda: One must observe the stringencies of where he comes from and where he is staying!

Rami: I ate them outside the Techum.

Rishonim

Rambam (Hilchos Ta'anis 1:15): If one went from a place where they do not fast to a place in which they fast, he fasts with them. If one went from a place where they fast to a place where they do not fast, he fasts and completes the fast. If one forgot and ate and drank, he should not show off in front of the Tzibur, and he should not indulge himself in pleasures.

Rosh (Ta'anis 1:7): If one went from a place where they fast to a place where they do not fast, he fasts and completes the fast, because he intends to return.

Ran (Ta'anis 2b DH Al): He should not show in front of them (that he ate), for this arouses envy.

Mordechai (Ta'anis 621): If one went to a place where they fast (he completes the fast). Riva says that this is even if he intends to return. However, since he did not accept the fast, if he left the Techum he may eat. He need not complete the fast. He derived this from the Beraisa. The Seifa does not say that he must complete the fast like the Reisha says.

(Note: in his text, the Seifa discusses one who went to a place where they fast, unlike our text.)

Mordechai: Also Chulin 110a connotes that outside the Techum is permitted. Rami ate udders outside the Techum. Avi ha'Ezri permits eating even in the city, in a room where no one will see him. He learns from Rabah bar bar Chanah. He intended to return, and even so he covered up De'asra so Bnei Bavel would not see him eating it. Fasts and Isurim are similar. Tosfos says that as long as he is in the city, he must join with the Tzibur in their pain.

Mordechai: If one forgot and ate and drank, he should not indulge himself in pleasures. I.e. he should not say 'since I forgot and ate and drank a little, I will finish my meal to satiation.' Rather, once he remembers he must cease eating. If one emits a foul smell because he ate garlic, should he eat more garlic and emit a stronger smell?! Even not in front of them, he may not indulge himself. Tosfos says so. Avi ha'Ezri forbids indulging in pleasures even in private, for this puts a glow on his face and it will be recognized. This seems correct. If not, the Seifa teaches what we already learned in the Reisha, that one should not show (that he ate) in front of them.

Poskim

Shulchan Aruch (OC 574:1): If one went from a place where they fast to a place where they do not fast, and he intends to return, he must fast all the fasts they accepted on themselves.

Beis Yosef (DH ha'Holech): B'Di'eved, if one ate and drank, he should not show off in front of them, for he is like a Chasan among mourners, and they will envy him.

Bach (1): Rashi says that he must complete the fasts that his (old) city accepted, because he gets the stringencies of his old and new places. This connotes even if he did not accept them, just the people of his city accepted them. This is only if he intends to return, like the Rosh says. Pesachim 51a proves that one (who intends to remain) follows the practices of his new place, whether this is a leniency or stringency. Tosfos says that one must complete the fast even if he will not return, i.e. if he accepted the fast. Then, it is a vow.

Tur: It seems that the Riva discusses one who goes to the new place on the day of the fast itself. The Sugya discusses one who went beforehand and accepted the fast. Then it is a full fast, and he may not eat even outside the Techum.

Question (Beis Yosef DH u'Mah she'Chasav Rabeinu): How does the Sugya connote this? If he accepted the fast, what is the Chidush? Obviously, he must complete the fast! Perhaps one might have thought that he accepted to fast only as long as he is with them.

Taz (1): This is difficult. The Seifa says 'if one forgot and ate and drank, he should not show this in front of them.' If he accepted the fast, obviously he must complete the fast. Why is this only 'in front of them'? Also, if so, also the Reisha (he went to a place where they do not fast) discusses one who accepted the fast. If so, why does the Tur say that it is only if he intends to return? Since he accepted to fast, he is obligated even if he does not intend to return! Rather, the Tur's words 'he accepted to fast' do not mean an explicit acceptance. Rather, he was with the Tzibur when they decreed the fast. It took effect on him, for the Tzibur decreed on everyone who pertains to them. It applies to him only because he intended to return. In the Seifa, he was in the new city when they decreed the fast on everyone there. The Tana teaches that even though he intends to return, he must include himself with them. Their decree applies to him, since he is there on the day of the fast. (If he did not fast,) there would be strife. The Tzibur has power to decree to avoid strife. This is why the Tur says that the Riva permits eating outside the Techum, or in a room, for one who came on the day of the fast. He was not there when they decreed yesterday, so the only concern is for strife. He must fast only in front of them. If he was there when they decreed, the decree does not apply to him totally, since he intends to return. Even so, it forbids him to eat even not in front of them, since there is concern for strife. If he forgot and ate, he may not show this, since the whole purpose is to avoid strife. The Tur learned that he left on the day of the fast from the Reisha (he left a place where they fast). Surely it would not apply to him if he is not there on the day of the fast! Therefore, also the Seifa discusses one who was there when they decreed.

Gra: Chulin 18b shows that it depends on intent to return.

Mishnah Berurah (1): Since he intends to return, letter of the law he must accept all stringencies of his old place. This is if he was there when they accepted the fast. This refers to a fast for Teshuvah, or so afflictions will not come. If it was for an affliction that passed, e.g. Sivan 20 in Polin, it does not need (being there at the time of the) acceptance. It is a proper Ta'anis Tzibur, so it applies to anyone who intends to return.

Kaf ha'Chayim (4): If he went from a place where they fast the 13 fasts for rain (and intends to return), he must fast, even if he was there only at the beginning.

Shulchan Aruch (2): If one went from a place where they do not fast to a place where they fast, he fasts with them, even if he intends to return.

Mishnah Berurah (2): Even if he ate before coming here, he must complete the fast with them.

Sha'ar ha'Tziyon (4): If he is weak, he may eat in private.

Magen Avraham (2): Regarding other matters of eating, one who intends to return may eat in private, even in the city. Here is different, for he must join with the city in their pain. Therefore, if the fast is for an affliction that already passed, he may eat in private if he intended to return. This requires investigation.

Mishnah Berurah (3): Eliyahu Rabah is stringent even about an affliction that passed.

Shulchan Aruch (ibid.): However, since he did not accept the fast, if he left the Techum of the city, he may eat and he need not complete the fast.

Gra (DH u'Mihu): Here it says 'he fasts with them' to teach that this is only when he is with them. We learn from Chulin 110a.

Sha'ar ha'Tziyon (5): The Bach and Eliyahu Rabah permit even if he does not intend to return. They learn from the Ran, who says that Rami was permitted to eat udder outside the Techum even if he did not intend to return. This is difficult. The Shulchan Aruch (496:3) says that if he does not intend to return, once he entered the Techum he becomes like a resident, and he is forbidden even outside the Techum. [However, it is difficult to say that Rami never entered the Techum and people cast their udders outside the Techum.] Perhaps a fast is more lenient, since he was not in the city when they accepted, so he must fast only to avoid strife, therefore he is permitted outside the Techum.

Kaf ha'Chayim (8): A weak person may be lenient like the Bach.

Rema: This is only if he arrived on the day of the fast. If he was there from the day before, he must accept the fast and complete it.

Shulchan Aruch (3): If one forgot and ate and drank, he should not show in front of them that he ate, and he should not indulge himself in pleasures, saying 'since I ate a little, I will eat a lot.'

Rema: Even not in front of them, he may not indulge himself.

Magen Avraham (4): The Mordechai brings from Riva that he may eat in private, but he may not indulge himself. However, the Beis Yosef rules like Tosfos, that one may not eat at all, If so, why did the Rema need to write that he may not indulge himself? He discusses even outside the Techum, for if he comes to the city his face will beam.

Gra (DH v'Afilu): The Rema teaches like the Riva. 'He may not indulge himself' must discuss not in front of them, for if not, the Reisha teaches this.

Kaf ha'Chayim (10): Likewise, he should not eat garlic or other things that give off smell, lest it be evident that he ate.

Mishnah Berurah (7): Some Acharonim explain simply, that one may not indulge himself (in matters other than eating).