Question: I am travelling to Israel on asara b’teves. Can I end my fast when it ends in Israel or do I need to wait until the fast ends in England?
Answer: R’ Shmuel Wosner (Shevet Halevi 7:76; 8:261:2) writes that as there are some poskim (Tosafos, Avoda Zara 34a; Vilna Gaon OC 562:1; Aruch Hashulchan OC 562:9) who hold that the minor fasts (asara b’teves, shiva asar b’tammuz and taanis Esther) end at shekia, one who started their fast earlier due to travel may rely on these opinions and end their fast at shekia. One travelling from Israel to America, for example, who found their taanis significantly lengthened, would be able to end their fast early if necessary, based on the timing of their departure city. One doing so must only eat what’s necessary, and not eat a proper meal until the fast is over in their arrival city. R’ Ephraim Greenblatt (Rivevos Ephraim 8:176) writes that one doing so should only eat in a quiet area and not in public where they’ll be seen by others. R’ Wosner doesn’t, however, address the scenario of one travelling eastwards who has their taanis shortened.
R’ Moshe Sternbuch (Teshuvos Vehanhagos 4:122) writes that we are always lenient for these three minor fasts. Thus, whether one was travelling from Israel to England or England to Israel, they would end their fast at the earlier Israeli time. He quotes R’ Dovid Menachem Munish Babad (Chavatzeles Hasharon 1:YD:47) who writes that just as a boy turns bar mitzva when he turns 13 and we don’t need to calculate that it has been 13 complete years based on the location where he was born, so too, we don’t need to ascertain when a person travelling began performing other mitzvos.
R’ Moshe Feinstein (Igros Moshe OC 3:96), however, writes that one fasts according to where they’re located. Thus, one travelling on a taanis will end their fast when it is nacht in their arrival city, regardless of whether that lengthens or shortens their fast. This would even apply to one travelling on tisha b’av as there is no requirement to specifically fast for twenty four hours.
In conclusion, while there is debate regarding one travelling westwards, one travelling from England to Israel on asara b’teves may end their taanis when it is nacht in Israel even though they have shortened their fast by travelling.