“Question: If I forgot to say al hanissim in birkas hamozon on Purim do I need to repeat birkas hamozon?
Discussion: The Shulchan Aruch (Orach Chayim Siman 695 se’if 3) rules that al hanissim is recited on Purim in the second brocho of birkas hamozon. The Magen Avraham (ibid s.k. 9) quotes several opinions (Shelah in Maseches Megillah, Maharshal, and Mateh Moshe) that maintain that if one forgot to say al hanisim in birkas hamozon, one needs to repeat birkas hamozon because there is an obligation to eat. The Magen Avraham queries this opinion (because tefilla is no less an obligation, and still if one forgets to recite al hanissim during tefilla he is not required to repeat the tefilla) but nonetheless concludes that if one has already eaten one meal during Purim [and thus has already fulfilled his basic obligation of mishteh] and during a subsequent meal he forgets to recite al hanissim, he does not need to repeat birkas hamozon.
It appears that the Magen Avraham agrees in theory to the opinions that maintain that if one forgot to say al hanisim in birkas hamozon one needs to repeat birkas hamozon, but limits this to the obligatory mishteh on Purim. Because there is no obligation to eat more than one mishteh on Purim, there is no requirement to repeat birkas hamozon if al hanissim was omitted during subsequent se’udos.
The Taz (Siman 695 s.k. 2) also quotes the opinion that maintains that if one forgot to say al hanisim on Purim, one needs to repeat birkas hamozon; however, the Taz (in a similar vein to the Magen Avraham) also limits this to the first se’udo that one eats on Purim day – even if one ate the se’udo in the morning.
The Pri Megodim (Siman 695 Mishbetzos Zohov s.k. 2) points out that according to the Magen Avraham (ibid) there is no obligation to eat bread on Purim, but according to the Taz (ibid) there is an obligation. Based on this, it would appear that if one ate a meal on Purim without eating bread, and subsequently ate a bread meal and forgot to say al hanissim in birkas hamozon, then according to the Magen Avraham birkas hamozon should not be recited again (because one already fulfilled his se’udo obligation and therefore the subsequent se’udo is not obligatory), and according to the Taz birkas hamozon should be recited again (since one has not yet fulfilled his se’udo obligation because he did not eat bread, the current meal is obligatory and thus al hanissim needs to be recited in birkas hamozon).
The Mishna Berura (Siman 695 s.k. 15) quotes the two opinions about this scenario; those who rule that one does need to repeat birkas hamozon, and those who rule that one does not need to repeat birkas hamozon. The conclusion of the Mishna Berura is that sofek brochos l’hokel [and thus birkas hamozon does not need to be repeated]. The Mishna Berura emphasizes that the machlokes only pertains to the first se’udo that one eats on Purim; regarding the subsequent meal(s) all agree that if one forgot to say al hanissim in birkas hamozon then one does not need to repeat birkas hamozon [and the fact that this later se’udo is referred to as “se’udas purim” is of no relevance]. In lieu of this, towards the end of birkas hamozon one can say the designated “horachamon”.
Conclusion: Although there is considerable discussion regarding one who forgot to say al hanissim during birkas hamozon on Purim, the conclusion of the Mishna Berura is that one should not repeat birkas hamozon.
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