We have finished siman 20, where the Chayei Adam discussed drinking from the kos of birchas hamazon after seudah shlishis. The question comes up regarding sheva brachos at seudah shlishis, where the seudah is not finished until bain hashmashos or tzais. The Chayei Adam already wrote that if it is bain hashmashos, one can be meikil and drink the kos. The Mishnah Berurah follows the Chayei Adam in this regard. Thus, if the sheva brachos finish during bain hashmashos, there is no question that the party can drink from the kos of birchas hamazon and the kos of sheva brachos.
The next question is what to do when they do not finish until tzais. Since they do not normally recite birchas hamazon with a kos, can they drink it? If they will drink it, who should drink it? Rav Moshe concludes that only the chosson and kallah should drink, and the person who lead birchas hamazon should not drink.
It appears from the teshuva that the main kos is the kos of sheva brachos, and once they are drinking from the kos of sheva brachos, we extend the heter to allow the chosson and kallah to drink from the kos of birchas hamazon as well. However, the minhag is to be meikil that the person leading birchas hamazon drinks, in addition to the chosson and kallah.
To understand this minhag, we need to understand why we use two kosos for sheva brachos. Tosfos in Pesachim explains that it is based on the concept of ain osim mitzvos chavilos chavilos, it is not appropriate to pile on mitzvos and fulfill them together (see more in shiurim 1384-1388). The Gemara questions how it is possible that on Motzei Shabbos which is Yom Tov we use the same kos for kiddush and havdalah. The Gemara concludes that it is not an issue only because kiddush and havdalah are one concept. Tosfos points out that kiddush and havdalah are different than birchas hamazon and sheva brachos, where, since they are not the same concept, one must use two kosos.
Taking this point a step further, the appropriate time to fill the second kos is a machlokes. The Magen Avraham understands that based on Tosfos, one should not fill the second kos at the same time as the first, because of the concept of ain osin mitzvos chavilos chavilos. However, the Rambam holds they can be filled at the same time.
Thus, the kos for birchas hamazon is separate from sheva brachos. If so, even if there is reason to drink the kos of sheva brachos, why would it extend to the kos of birchas hamazon? We will answer this question in the upcoming shiur, be’ezras Hashem.
Summary
If seudah shlishis sheva brachos finish during bain hashmashos, one may drink from the kosos. If they do not finish until after tzais, Rav Moshe holds that only the chosson and kallah should drink, but the minhag is for the person leading birchas hamazon to drink as well.
