“Question: What is the correct thing to do if an individual who is not fasting was already called up for an aliya on a ta’anis tzibbur?
Discussion: As previously discussed, the Shulchan Aruch (Orach Chayim Siman 566 se’if 6) rules that a non-fasting individual should not receive an aliya on a ta’anis. Also, if a Kohen is in Shul and is not fasting, he should leave Shul and a Yisroel should be called up instead of him. [Obviously, if there are other Kohanim in Shul there is no need for the non-fasting Kohen to leave but he should ensure that another Kohen is called up instead of him.]
The Mishna Berura (Siman 566 s.k. 20) adds that even if one is currently fasting but not intending to complete the ta’anis, should also not receive an aliya.
Regarding a situation where a non-faster was already called up for an aliya on a ta’anis, the Magen Avraham (Siman 566 s.k. 8) quotes a machlokes between the Beis Yosef (Siman 135 se’if 5; it appears that it is implied, but not explicit in the Beis Yosef) and Bach (Orach Chayim Siman 566 d.h. “d’shliach tzibbur”); the Beis Yosef maintains that even ex post facto a non-faster cannot have an aliya; the Bach posits that in this situation the non-faster can have the aliya. The conclusion of the Magen Avraham himself is that there is no prohibition for the non-faster to have the aliya (b’dieved).
The Taz (Siman 566 s.k. 7) however, argues that for a non-faster to have an aliya on a ta’anis is a concern of brocho l’vatolo; consequently, even if a non-faster was already called up he may not have an aliya. (The Taz also writes that even the Tur who allows one who is not fasting to be the shaliach tzibbur agrees that he cannot receive an aliya; because for one who is not fasting to receive an aliya is considered a brocho l’vatolo.)
The Mishna Berura (ibid s.k. 21) also strongly advises an individual who is not fasting to avoid being in Shul during keriyas hatorah to prevent the predicament of being called up for an aliya while not fasting. If it so happened that a non-fasting individual was called up for an aliya, the Mishna Berura rules that if the individual in question is a Talmid Chochom and it will cause a chillul Hashem if it becomes evident that he is not fasting, we can rely on the lenient opinions [i.e. the Bach and Magen Avraham] ex post facto and he may have the aliya.
From the specific case the Mishna Berura brings of a Talmid Chochom and chillul Hashem it appears that in all other situations even ex post facto it would not be acceptable for a non-faster to have an aliya on a ta’anis. The exception to this is during Shacharis, if the ta’anis coincides with a Monday or Thursday. This is because the Magen Avraham (s.k. 8) writes that in this situation a non-faster may receive an aliya because there would be keriyas hatorah on this day even without the ta’anis [and the keriyas hatorah is applicable to the non-faster as well].
Conclusion: An individual who is not fasting on a ta’anis tzibbur, or even intending not to complete the ta’anis should ideally not receive an aliya.
If this individual was accidentally called up, then if this occurred during Shacharis on a Monday or Thursday, the non-faster may receive the aliya. If this occurred during Shacharis on a different weekday, or during Mincha on any day of the week, then the non-faster may not have the aliya. The only exception to this is if the non-faster is a Talmid Chochom and his not having the aliya will result in a chillul Hashem then he may have the aliya.
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