Accepting a Fast on the Yahrzeit Day

Is it necessary to accept the fast during the Mincha before the Yahrzeit day?

If one is accustomed to fasting every year, the custom is to accept it during the Mincha before, but it is not obligatory, and if he did not accept, he may fast. However, if he is not accustomed to fasting every year and wishes to fast, he must accept it during the Mincha before, and if he did not accept, it is not considered a fast. Nevertheless, he should fast because he thought he accepted upon himself a partial fast.

If one has done so even once, he is not required, as written in Chochmat Adam, Sha'ar HaSimcha, Klal 171:11: "It is a mitzvah to fast on the day one's father and mother died every year" (Rama, Siman 376:4). And if he has already fasted once, it is assumed that he intended to fast all his life, and it becomes a vow which is obligatory by Torah law, and he must fast, etc." Therefore, if he is accustomed to fasting every year, he is not required to accept it during the Mincha prayer, but the custom is to accept it during Mincha, although it is not obligatory. If he does not do so, he must accept it during the Mincha before, and if he did not accept, it is not a fast, as explained in the Gemara, Taanit 12a: "Shmuel said: Any fast that was not accepted beforehand is not a fast. And if he fasts, what then? Raba bar Shila said: It is like a bellows filled with wind. When does he accept it? Rav said: during Mincha, and Shmuel said: during the Mincha prayer." And the law follows Shmuel, as ruled in Shulchan Aruch, Orach Chaim, Siman 562:5: "Any fast that was not accepted beforehand is not a fast. Note: to pray 'Aneinu', but if he is obligated to fast and fasted thus, he has not fulfilled his vow." And what I wrote that he must fast is explained in Biur Halacha, Siman 562.