We are beginning siman 20. The Chayei Adam writes that we have learned that once a person begins seudah shlishis, they can continue their seudah even after the zman for tzais. Once they are finished, they recite birchas hamazon, and, according to the Chayei Adam, may drink the kos of brichas hamazon. However, we know that one cannot drink anything until making havdalah. To explain, we have learned that it is a machlokes whether birchas hamazon requires a kos. According to the opinion that it does require a kos, the drinking is part of the chiyuv birchas hamazon, which is part of the meal. If so, the kos is an extension of the meal, so it is muttar to drink it before havdalah. However, this thought process only applies if one holds that birchas hamazon requires a kos. If one is not careful about always reciting birchas hamazon on a kos, they are following the opinion that birchas hamazon does not require a kos, meaning, the kos is not part of the meal. If so, it would be assur to drink it. This is the source for the minhag to leave the kos of birchas hamazon for havdalah, such that one fulfills both the mitzvah of birchas hamazon on a kos and havdalah through that kos. However, one who is makpid to always recite birchas hamazon on a kos should drink the kos immediately after birchas hamazon, before making havdalah. The Chayei Adam clarifies that the above reasoning to prohibit one from drinking the kos only applies once it is certainly after tzais. If it is still bain hashmashos, even if one is not always makpid to use a kos for birchas hamazon, they may drink the kos immediately following birchas hamazon. The Mishnah Berurah (siman 299:sk14) quotes the Chayei Adam and follows him. The Chayei Adam explains himself in the parenthesis. He writes that the opinion that one can only drink the kos if they always recite birchas hamazon on a kos is brought by the Magen Avraham. The Taz disagrees, and holds that any time one recites birchas hamazon they may drink the kos, even if they do not normally recite birchas hamazon on a kos. Therefore, the Chayei Adam paskens that in cases of safeik (i.e., bein hashmashos) one may rely on the Taz. The Shaar Hatziyun points out that any time a person recites birchas hamazon on a kos, even if it is not a chiyuv, it is a mitzvah min hamuvchar. Therefore, if one is allowed to drink the kos, one should do so. Summary
- One who is careful to always recite birchas hamazon on a kos may drink the kos after seudah shlishis, even if it is past shkia.
- Even if one is not always careful to use a kos, they may drink the kos if it is still bain hashmashos.
- It is always mitzvah min hamuvchar to use a kos for benching.