Halakhic Positions of Rav Joseph B. Soloveitchik
by R. Aharon Ziegler
After the conclusion of Shabbat we are not permitted to eat before reciting Havdalah over a cup of wine (Havdalah al HaKos). This is true even if we have already recited Havdalah during Arvit, which serves to remove the prohibition of performing forbidden work.
While we find in many contexts a similar prohibition against eating before performing a Mitzva in order not to delay its performance, the prohibition of eating before Havdalah Al HaKos is unique, as it is connected to the essence of the Mitzva itself. For example, the Gemara (Pesachim 106b) quotes an opinion that if we inappropriately eat before reciting Havdalah Al HaKos, we have not only violated the prohibition of not to eat, but we can no longer recite Havdalah at all. This stands in contrast to other Mitzvot in which we do not find that eating before their fulfillment would invalidate the Mitzva. For example, it is forbidden to eat before Davening (Praying) Shacharit (Brachot 10b), but if we have eaten, we can (and must) still recite the morning prayers.
This discrepancy between Havdalah and other Mitzvot arises from the fact that there are two aspects to Havdalah, parallel to the two aspects of Kiddush. Kiddush in the Amida of Shabbat evening ushers in Shabbat and its restrictions against doing Melacha, while Kiddush al HaKos introduces the Se’udot of Shabbat. Similarly, the Havdalah in Arvit ends Shabbat and makes work permissible, while Havdalah al HaKos concludes the Se’udot of Shabbat and marks the beginning of the meals of the week. Thus, eating before Havdalah al HaKos not only delays the fulfillment of the Mitzva, but undermines its essential purpose; to serve as the conclusion of the Shabbat meals and the commencement of the weekday meals. Therefore, there is an opinion in the Gemara that if we improperly eat before reciting Havdalah al HaKos, there is no longer any purpose in reciting it at all. Furthermore, Havdalah al HaKos not only marks the end of the meals of Shabbat, but concludes all of Kavod and Oneg Shabbat. Thus, even though Yom Kippur has no Mitzva of Se’udot Yom Tov, we still recite Havdalah al HaKos at its conclusion to mark the end of the Mitzva of Kavod Yom Tov that we fulfill through wearing of Yom Tov attire.
This also explains the ruling of the Magen Avraham (262:2) that we should not change out of our Shabbat clothing until after Havdalah al HaKos. Even though there is no more prohibition to perform work after that Havdalah in Tefilla, it is the Havdalah al Hakos which marks the end of all aspects of Kavod and Oneg Shabbat.
(Source: Shiurim LeZecher Abba Mari, vol 2, p. 131)