Yahrzeit on Friday and usually prays Mincha at Plag.

Someone who usually prays the Friday Maariv at Plag Mincha and has a yahrzeit for his mother on Friday, until what time should he fast?

According to the Sephardic custom: one must specify at the time of accepting the fast that he only accepts it until after Maariv at Plag Mincha, and if he did not specify, he must fast until the stars appear. According to the Ashkenazi custom: if the first time the yahrzeit fell on a Friday he completed his fast until after the stars appeared, he must continue to do so every time the yahrzeit falls on a Friday. If this is the first yahrzeit after the passing or the first time he wishes to fast, he can fast until after Maariv, and it is good to stipulate at the time of accepting the fast that he only accepts it until after Arvit. Even if he did not stipulate, it is forbidden to fast until the stars appear. If this is not the first yahrzeit he fasted, but it is the first time the yahrzeit falls on a Friday, when in distress, he can rely on those who are lenient to eat while the day is still large after Arvit.

It is written in the Shulchan Aruch, Orach Chaim, Siman 249, Seif 4: "If he accepted upon himself to fast on Friday, he must fast until the stars appear, unless he specified at the time of accepting the fast to complete it after the congregation finishes their prayer." And the Rema wrote: "There is an opinion that one should not complete, but immediately upon leaving the synagogue, he may eat." The Rema also wrote in Yoreh Deah, Siman 402, Seif 12: "If this fast falls on the eve of Shabbat, it has the same law as other fasts. And see in Orach Chaim, Siman 249. However, if the first time he completed, he should continue to do so all his life." And it is written in Biur Halacha, Siman 249, Seif 4: "See in Mishnah Berurah that according to this opinion, it is not proper to wait from the time of coming from the synagogue until the stars appear if he desires to eat and is delayed because of the fast. Therefore, if he stipulated or even did not stipulate, but this is the first fast, it is forbidden to complete after Maariv before the stars appear. What I wrote, that the first time it fell on the eve of Shabbat, in a place of distress, one can rely on those who are lenient, so it is written in Mishnah Berurah, Siman 249, Seif Katan 22: "If initially, when he began to fast on the day his father or mother died, or on the eve of the month, and other mentioned fasts fell on a weekday, and on a weekday they complete, and therefore there are opinions that even if it later falls on the eve of Shabbat, he must also complete until the stars appear, as it is assumed that he intended to always behave so, unless he specified that he does not intend to always behave so, and there are those who disagree with this and hold that it is assumed he did not accept upon himself to complete even on the eve of Shabbat, and therefore one who is distressed can rely on those who are lenient and eat immediately after leaving the synagogue, even if the day is still large."