Kiddush
7. Friday night Kiddush. It is a mitzvas asei to sanctify the Shabbos verbally upon its entrance, as the posuk says, “זכור את יום השבת לקדשו ” (גמ' פסחים דף ק''ו ע''א ). Some Rishonim hold that using wine for Kiddush is also d’Oraisa (רש''י/ריב''ם נזיר דף ג' ע''ב, ר''ח פסחים דף ק''ז ע''א ); others hold while verbal Kiddush is d’Oraisa, using wine for Kiddush is only d’Rabanan (תוס' נזיר שם, רמב''ם פכ''ט שבת הי''א, רמב''ן עה''ת שמות כ', ח'). Yet others hold one must make Kiddush with a cup of wine mid’Oraisa, but there is no chiyuv d’Oraisa to drink it (רוקח סי' נ''ב ).
8. Shabbos day Kiddush. The kos shel bracha at Kiddush Shabbos day is different than other kosos shel bracha, as there is nothing in the established nusach other than the bracha of Borei Pri HaGafen. In other words, the essence of Kiddush is making a Borei Pri HaGafen on a cup of wine (רבינו דוד פסחים דף ק''ה ע''ב ). Some explain it is still called Kiddush since the Shabbos is sanctified through making a bracha on a cup of wine, as that action makes it clear it is Shabbos and distinguishes it from the other days of the week (רבינו יונה הובא בר בינו דוד, רמב''ם פכ''ט שבת ה''י ).
9. Yom Tov Kiddush. Kiddush on Yom Tov must also be with a kos. Most poskim hold Yom Tov Kiddush is only d’Rabanan, so the chiyuv to use a kos is also only d’Rabanan (מג''א סי' רע''א סק''א ). However, here too, some hold Kiddush is d’Oraisa (א''ר בשם הלכות גדולות ), and some cite an opinion that Kiddush by day is also d’Oraisa (מג''א סי' תקצ''ו סק''ו בשם מהר''ם ).
Havdala
10. Havdala on Motzei Shabbos must be with a kos. Most Rishonim hold the kos is only a mitzva d’Rabanan (רמב''ם פכ''ט שבת ה''ו, טושו''ע או''ח סי' רצ''ו ). However, here too, some say it is d’Oraisa (תוס' רי''ד פסחים דף ק''ו ).
Daled Kosos
11. Chazal instituted a chiyuv on the first night of Pesach to say four brachos on four cups of wine (גמ' פסחים דף צ''ט ע''ב ). The Yerushalmi says the four kosos correspond to the four kosos mentioned in the sar hamashkim’s dream and Yosef’s interpretation (ירושלמי פסחים פ''י ה''א ).
Birkas HaMazon
12. There is a machlokes Rishonim whether one must say Birkas HaMazon with a kos. There are three primary opinions.
13. A kos is required even when bentching alone. Some Rishonim hold one must use a kos for Birkas HaMazon even when bentching alone (תוס' פסחים דף ק''ה ע''ב ד''ה ש''מ, רא''ש פ''י סי' י''ד, רבינו יונה ברכות דף ל''ז ע''ב, טור סי' קפ''ב, מהרש''ל הובא במג''א סק''ב, ב''ח, א''ר סק''א ).
14. A kos is required with a zimun. Others hold one must use a kos when bentching with a zimun, but not alone (הג' מיימונית שם, ב''י שם, חי''א כלל מ''ו אות ה', קיצושו''ע מ''ה א'). A hint to this is found in the posuk “ובגפן שלשה שריגים ” (פ' וישב ) (סדר היום כונת סדר אכילה ט''ו ).
15. Kos is never required. Yet others hold Birkas HaMazon never requires a kos (רמב''ם פ''ז ברכות הט''ו, רשב''א ברכות דף נ''ב ע''א ). Nevertheless, bentching with a kos is still the best way to perform the mitzva (רבינו מנוח והר''ן הובא בב''י ).
16. The halacha. The Mechaber cites these three opinions without giving a conclusive psak. Some say he means to pasken like the third opinion, which he cites last, that no kos is required (כף החיים ). The Rama adds that while there is no chiyuv to use a kos, bentching with a kos is the best way to do the mitzva.
17. The minhag. Some say the prevalent minhag is not to make an effort to use a kos for Birkas HaMazon even when there is a zimun (שו''ע הגר''ז ס''א ). However, if one has wine in the house and he can easily bentch with a kos, certainly everyone would agree it is best to bentch with a kos if there is a zimun (מ''ב סי' קפ''ב סק''ד וסקי''ד ).
18. Others explain that the reason the prevalent minhag is not to make an effort to use a kos shel bracha is because wine was not so readily available, so they would follow the third opinion, that Birkas HaMazon does not require a kos. Some explain that even though wine is readily available today, since the minhag for centuries was to pasken that a kos is not necessary, that became the established minhag today as well (אג''מ יו''ד ח''ג סי' נ''ב אות ג').
19. Yerei Shomayim. Even if we can justify those who do not use a kos for Birkas HaMazon with a zimun, a yerei Shomayim should bentch with a kos, especially now that wine is available (ערוך השלחן ס''ג ). This is even more true today, when grape juice is readily available in every house, and drinking grape juice does not cause people discomfort. That being the case, it is a situation of “having wine in the house,” in which case one should use it for Birkas HaMazon (מ''ב סקי''ד, ס' החדש והנפלא כוס ישועות להגרמ''י בלייער שליט''א י"א, ט').
